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Marco Cantů Essential Pascal

1. In both cases you are defining a constant that is a value you don t change during program execution The difference is only in the implementation A string constant defined with the resourcestring directive is stored in the resources of the program in a string table To see this capability in action you can look at the EPConstants example which includes the following code resourcestring strAuthorName Marco Cantu strBookName Essential Pascal begin writeln strBookName strAuthorName The output of the two strings appears with a space in between The interest ing aspect of this program is that if you examine it with a resource explorer there is one available among the examples that ship with Delphi you ll see the new strings in the resources This means that the strings are not part of Marco Cantu Essential Pascal Chapter 3 Types Variables and Constants 33 the compiled code but stored in a separate area of the executable file the EXE file In short the advantages of using resources are more efficient memory han dling performed by Windows and a better way of localizing a program translating the strings to a different language without having to modify its source code As a rule of thumb you should use resourcestring for any text that is shown to users and might need translating and internal con stants for every other internal program string like a configuration file name Data Type
2. Integer V 2 V I N S t Vj Writing this kind of code is risky to say the least If the string contains a number everything works If not an exception is raised Again you can write similar code but without a compelling reason to do so you shouldn t use the Variant type stick with the traditional Pascal data types and type checking approach In Delphi and in the VCL Visual Component Library variants are basically used for OLE support and for accessing database fields Variants in Depth Delphi and FreePascal include a variant record type TVarData which has the same memory layout as the Variant type You can use this to access the actual type of a variant The TVarData structure includes the type of the Variant indicated as VType some reserved fields and the actual value The possible values of the VType field correspond to the data types you can use in OLE automation which are often called OLE types or variant types Here is a complete alphabetical list of the available variant types varArray varBoolean varByRef varCurrency varDate varDispatch varDouble varEmpty varError varInteger varNull varOlestr varSingle varSmallint varString varTypeMask varUnknown varVariant You can find descriptions of these types in the Values in variants topic in the Delphi Help system There are also many functions for operating on variants that you can use to make specific type conversions or to ask for information about the type of a v
3. a5 writeln C Stri 2 a writeln stri StringStatus Str1 writeln str2 StringStatus Str2 This is the corresponding output Strl Address 13419112 Length 5 References 1 Value Hallo Str2 Address 13419088 Length 5 References 1 Value Hello You can freely extend this example and use the StringStatus function to explore the behavior of long strings in many other circumstances 45 Keep in mind that this internal information about offsets might change in any future version of Delphi there is also no guarantee that similar undocumented features will be maintained in the future Marco Cantu Essential Pascal 94 Chapter 7 Handling Strings Delphi Strings and Windows PChars Another important point in favor of using long strings is that they are null terminated This means that they are fully compatible with the C language null terminated strings used by Windows A null terminated string is a sequence of characters followed by a byte that is set to zero or null This can be expressed in Delphi using a zero based array of characters the data type typically used to implement strings in the C language This is the reason null terminated character arrays are so common in the Windows API func tions which are based on the C language Since Pascal s long strings are fully compatible with C null terminated strings you can simply use long strings and cast them to PChar when you need to pass
4. Date and Time Pascal implementations typically use floating point data types to handle date and time information To be more precise both Delphi and FreePascal define a specific TDateTime data type These are a floating point type because the type must be wide enough to store years months days hours minutes and seconds down to millisecond resolution in a single variable Dates are stored as the number of days since 1899 12 30 with negative values indicating dates before 1899 in the inte ger part of the TDateTime value Times are stored as fractions of a day in the decimal part of the value TDateTime is not a predefined type the compiler understands but it is defined in the system unit as type TDateTime type Double Using the TDateTime type is quite easy because Delphi includes a number of functions that operate on this type Here you can find a list with some of these functions Now Returns the current date and time into a date time value Date Returns only the current date Time Returns only the current time 17 GNU Pascal adopts a different approach to storing dates and times It defines a TimeStamp record more about records later which stores each element of the date and time separately It has a comparatively limited number of routines for handling dates and times as individual elements of the date and time such as Hour can be directly accessed Whilst this approach is simpler and slightly faster the pena
5. dures which are much more efficient but cannot be used with component properties of the set type Include Style fsBold Conclusion Now that we know the basic layout of a Pascal program we are ready to start exploring its meaning Well start by looking at predefined and user defined data types then we ll start using keywords to form programming statements Marco Cantu Essential Pascal 28 Chapter 2 Coding in Pascal Marco Cantu Essential Pascal Chapter 3 Types Variables and Constants 29 Chapter 3 Types Variables And Constants The original Pascal language introduced some new notions which have now become quite common in programming languages The first then revolution ary notion is that of data type The type determines the values a variable can hold and the operations that can be performed on it The concept of type is stronger in Pascal than in C where the arithmetic data types are almost interchangeable and much stronger than in the original versions of BASIC which had no similar concept That s why programmers refer to Pascal as a strongly typed language Marco Cantu Essential Pascal 30 Chapter 3 Types Variables and Constants Variables Pascal requires all variables to be declared before they are used Every time you declare a variable you must specify a data type Here are some sample variable declarations var value Integer IsCorrect Boolean A B Char The var keyword ca
6. en wikipedia org wiki De_Morgan 27s_laws Marco Cantu Essential Pascal Chapter 5 Statements 67 Random Number 192 Random Number 186 Random Number 263 Random Number 195 Random Number 347 Random Number 196 Random Number 379 Random Number 214 Random Number 437 Random Number 311 Random Number 531 Random Number 403 Random Number 583 Random Number 429 Random Number 660 Random Number 468 Random Number 683 Random Number 515 Random Number 689 Random Number 608 Random Number 751 Random Number 628 Random Number 775 Random Number 722 Random Number 798 Random Number 776 Random Number 888 Random Number 824 Random Number 910 Random Number 889 Random Number 948 Random Number 967 Random Number 970 Random Number 1062 Random Number 1019 Notice that not only are the generated numbers different each time but so is the number of items This while loop is executed a random numbers of times If you execute the program several times in a row you ll see that the output has a different number of lines You can alter the standard flow of a loop s execution using the Break and Continue system procedures The first interrupts the loop the second is used to jump directly to the loop test or counter increment continuing with the next iteration of the loop unless the condition is zero or the counter has reached its highest value 85 Two more system procedures Exit and Halt let you immediately return from the current fun
7. 116 Formatting placeholders 96 Forward Declarations 79 Free Pascal 14 x 20 uaar 56 FreePascal pala annata 3 Andi se eee eter n 26 CHR 26 ONE E E 31 Conversion riccio 31 I ion E 25 27 Rsclhude arn 27 EXPIeSSIONS ssscsessecesnseseenseeseevenses 24 Ischia lai 26p T cl de aset e 27 IR 26 Keywords ceti etre 23 TT matu 26 INO noted eee 25 EE 25 0 DE 26 Precedence of Operators 25 Een ln ng nire 25 Set Operators eene 27 e UE 26 Shrine 26 Malt titani atrata 30 Variables secto eebe e 30 Wiitelni uil a a teer Re 24 e EE 26 RAEE RATE ARRESE RE REA 25 Funto D ressonar a eCa e ums 71 Function poimter e 81 GetMeM cccccccecsssrsnensnsrsnsnsreenes 56 58 Get Dickount iii 116 Getlserhame ttt 94 107 Global memory eere 99 GNU Pascal eorpore tester eese 3 Haltii girella 67 Handles ettet eere 106 ST M 100 Hello World esses 15 Sr fina mua 38 54 ERRO I AI ER 61 Nicklaus Wirth Ordinal Trpes tte Parameter passing Index 139 Pat metets oerte an Constatine errena 74 QE seu et te EE Ee 74 R ference zione 73 Malte trees erede oto 73 Param OT ED ii 110 Pascali cnr 12 79 Pascal Strings eese 90 Pascal casing eese 18 PIE E 94 POINTS nate tectedeenesie Eege 56 lu TL
8. 9 write Low Integer write 9 write High Integer writeln write SmalliInt write 9 write Sizeof smallInt write 9 write Low Smallint write 9 write High SmallInt writeln write znt64 write 9 write Sizeof Int64 write 9 write Low Int64 write 49 write High Int64 writeln Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 37 write Char write 9 write sizeof char write 9 write ord Low Char write 9 write ord High CChar writeln readln end The code is somewhat repetitive and it could have been written in a much nicer way but I didn t want to introduce too many concepts at once This is the output slightly reformatted for clarity Integer 4 2147483648 2147483647 SmallInt 2 32768 32767 Int64 8 9223372036854775808 9223372036854775807 Char 1 0 255 The program uses three functions Sizeof High and Low The result of the Sizeof function is an integer indicating the number of bytes required to represent values of the given type The results of the last two functions are ordinals of the same type as that of the value supplied to them indicating the valid range of values represented by the type itself To display the range of the Char type the program con verts the character into its numeric representation using Ord as character 0 is non printable and char
9. 9 The EPVariables example has the variable declarations and the assignments listed in this section plus a couple of writeln statements to display something on screen Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 31 Of course it is often possible to convert the value of a variable from one type to another type In some cases the conversion is automatic but usually you need to call a specific system function that changes the internal representa tion of the data In Pascal you can assign an initial value to a global variable while you declare it For example you can write var value Integer 10 Correct Boolean True This initialization technique works only for global variables not for variables declared inside a procedure or function Constants Pascal also allows the declaration of constants allowing you to give meaning ful names to values that do not change during program execution To declare a constant you don t need to specify a data type but only assign an initial value The compiler will look at the value and automatically use its proper data type Here are some sample declarations from the EPConstants exam ple const Thousand 1000 Pi 3 14 AuthorName Marco Cantu Pascal determines the constant data type based on its value In the example above the Thousand constant is assumed to be of type Smal 1Int the smallest integral type which can hold it If you want to tel
10. Converts a floating point value to its string represen tation using the specified formatting Copies a floating point value to a string buffer using the specified formatting As the previous one copies the floating point value to a string buffer using the specified formatting Converts a Pascal string to a floating point value Converts a null terminated string to a floating point value Some of these routines work on the data types that we ll discuss in the fol lowing sections Notice that the table doesn t include routines for special types such as TDateTime or variant or routines specifically intended for formatting like the powerful Format and FormatFloat routines In recent versions of Delphi s Pascal compiler the Round function is based on the native implementation offered by the CPU This processor adopts the so called Banker s Rounding which rounds middle values such as 5 5 or 6 5 up and down depending whether they follow an odd or an even number Summary In this chapter we explored the basic notion of type in Pascal But the lan guage has another very important feature It allows programmers to define new custom data types called user defined data types which are covered in the next chapter 20 Andin the FreePascal Compiler Marco Cant Essential Pascal Chapter 4 User Defined Data Types 47 Chapter 4 User Defined Data Types Along with the notion of type one of the great new ideas i
11. So if you write a lt b and cC lt d the compiler will do the and operation first resulting in a compiler error So you should enclose each of the lt expressions in parentheses a b and c d Some of the common operators have different meanings when used with dif ferent data types For example the operator can be used to add two Marco Cant Essential Pascal Chapter 2 Coding in Pascal 27 numbers concatenate two strings make the union of two sets and even add an offset to a PChar pointer However you cannot add two characters as is possible in C Another unusual operator is div In Pascal you can divide any two numbers real or integers with the operator and you ll invariably get a real number result If you need to divide two integers and want an integer result use the div operator instead Here are two sample assignments this code will become clearer as we cover data types in the next chapter realval 123 12 intergerval 123 div 12 Set Operators The set operators include union difference intersection mem bership test i n plus some relational operators To add an element to a set you can make the union of the set with another one that has only the ele ments you need Here s a Delphi example related to font styles Style Style fsBold Style Style fsBold fsrtalic fsunderline As an alternative you can use the standard Include and Exclude proce
12. You cannot use an identifier outside its scope Here are some examples e Global hidden variables If you declare an identifier in the imple mentation portion of a unit you cannot use it outside the unit but you can use it in any block and procedure defined within the unit The memory for this variable is allocated as soon as the program starts and exists until it terminates You can use the initialization section of the unit to provide a specific initial value e Local variables If you declare a variable within the block defining a routine or a method you cannot use this variable outside that pro cedure The scope of the identifier spans the whole procedure including nested routines unless an identifier with the same name in the nested routine hides the outer definition The memory for this variable is allocated on the stack when the program executes the routine defining it As soon as the routine terminates the memory on the stack is automatically released e Global variables If you declare an identifier in the interface por tion of the unit its scope extends to any other unit that uses the one declaring it This variable uses memory and has the same lifetime as the first group the only difference is in its visibility Any declarations in the interface portion of a unit are accessible from any part of the program that includes the unit in its uses clause Variables of 52 In fact even the effect of the private keyword for a
13. operator effectively creating an alias a new variable referring to the same array in memory At this point however if you modify one of the arrays the other is affected as well as they both refer to the same memory area alias Array2 e Arrayl Marco Cant Essential Pascal Chapter 8 Memory 103 change one both change Array2 99 1000 show the other writeln IntToStr Arrayl 99 At this point the sample program does two more operations The first is an equality test on the arrays This doesn t tests the actual elements of the structures but rather the memory areas the arrays refer to checking whether the variables are two aliases ofthe same array in memory if Arrayl Array2 then Beep truncate first array Arrayl Copy Array2 0 10 The second is a call to the Copy function which not only copies data from one array to the other but also replaces the first array with a new one cre ated by the function The effect is that the Array1 variable now refers to an array of 11 elements so that asking for value 99 again will produce a memory error and raise an exception unless you have range checking turned off in which case the error remains but the exception is not displayed Summary This chapter introduced the role of different memory areas and covers dynamic arrays an important element for memory management The next Chapter will focus on bare bone Windows applications the following
14. similar formatting style Syntax Highlighting To make it easier to read and write Pascal code the Delphi editor and many others have a feature called color syntax highlighting Depending on the meaning in Pascal of the words you type in the editor they are displayed using different colors By default keywords are in bold strings and com ments are in color and often in italic and so on Marco Cantu Essential Pascal Chapter 2 Coding in Pascal 21 Reserved words comments and strings are probably the three elements that benefit most from this feature You can see at a glance a misspelled keyword a string not properly terminated and the length of a multiple line comment In Delphi you can easily customize the syntax highlight settings using the Editor Colors page of the Environment Options dialog box If you are the only person using your computer to look to Pascal source code choose the colors you like If you work closely with other programmers you should all agree on a standard color scheme I find that working on a computer with a different syntax coloring than the one I am used to is really difficult Error Insight and Coding Helpers Recent versions of the Delphi editor have many more features to help you write correct code The most obvious is Error Insight that places a red squiggle under source code elements it doesn t understand in the same fash ion a word processor marks spelling mistakes Other features lik
15. 38 Pretty Printing Yat da re avot 10 Printer Ea 130 Procedural Types esee 81 Procedir ennerien eI 71 eT 124 nO rea cri 66 Range Checking iii 50 Reali eni 39 Real Types ccce lia ee 39 R alAB eec ten As t eni 39 Record oiran 54 Reference count 93 Reference counting i 91 2 ecrire ea ie edere 78 R e oei snes iis na nines int nes inerse na eisor Entis 65 hog aiar 128 Resourcestring srscvriviericecias 32 Result TT 72 REWI EE 128 Pound eere neis 45p ROUNE aea A re a EE ER 71 RI e 37 Ry alesis ai eI ee ia 61 SCOPE ici 122 Semicolon iii 59p Seli EE 51 SetLength cin 92 101 SliortInt zie d eec ove e 34 ShortiString eni nee pete 90 Single remeare ed 39 SEO E 37 Slice MO 75 eT SY farai 34 Marco Cantu Essential Pascal 140 Index Source code cri 5 Stick PEE 100 Btatement ss sese sees e sees eenn 59 Statements enne 21 Stdcall sro glia Matia 79 108 lux 45 Sent bc iaia 89 159 EE 45 Nod 0h nana 45 St ToFl dt iii 46 Str KON G ei REEE ETT 45 e ST iii 49 Nec 38 SWAG 5 ccce etie ai 136 EK 16 Syntax Highlighting 20 ee e EE 43 T DIeBX oie Gert rere ees 48 TDateTime iii 41 TextFile dica done 127 130 TextToEl dt 4 ere
16. 38 Do not confuse the TVarRec record with the TVarData record used by the Variant type itself These two structures have a different aim and are not compatible Even the list of possible types is different because TVarRec can hold Delphi data types while TVarData can hold Windows OLE data types Marco Cantu Essential Pascal 78 Chapter 6 Procedures and Functions Result Args I VInteger vtBoolean if Args I VBoolean then Result Result 1 vtchar Result Result Ord Args I vChar vtExtended Result Result Args I VExtendedA vtString vtAnsiString Result Result StrToIntDef CC Args I vStringA 0 vtwidechar Result Result Ord Args I vwidechar vtcurrency Result Result Args I VCurrencyA end case end I ve added this code to the OpenArr example which calls the SumA11 func tion with the following code var X Extended Y Integer begin Y 10 X SumAll C Y Y K True 10 34 99999 writeln Format sumAT Y Y k True 10 34 99999 gt n x end You can see the output of this call below SumAll Y Y k True 10 34 99999 gt 10 217 34 Delphi Calling Conventions The 32 bit version of Delphi introduced a new approach to passing parame ters known as fastcall Whenever possible up to three parameters can be passed in CPU registers making the function call much faster The fast call ing convention used by defau
17. Although a forward procedure declaration is not very common in Pascal there is a similar case that is much more frequent When you declare a pro cedure or function in the interface portion of a unit more on units in the next chapter it is considered a forward declaration even if the forward key word is not present Actually you cannot write the body of a routine in the interface portion of a unit At the same time you must provide in the same unit the actual implementation of each routine you have declared 39 The same holds for the declaration of a method inside a class type that was automatically generated by the Delphi IDE as you added an event to a form or its components The event handlers declared inside a TForm class are forward declarations the code will be provided in the implementation portion of the unit Marco Cant Essential Pascal Chapter 6 Procedures and Functions 81 Procedural Types Another unique feature of Object Pascal is the presence of procedural types These are really an advanced language topic which only a few Delphi pro grammers will use regularly However since we will discuss related topics in later chapters specifically method pointers a technique heavily used by Delphi it s worth a quick look at them here If you are a novice program mer you can skip this section for now and come back to it when you feel ready In Pascal there is the concept of procedural type which is similar to t
18. EE 70 Chapter 6 Procedures and FunctionS sesesessecesescecescecescescescessescescescessessessessesesseeo 71 Pascal Procedures and Functionsg ii 71 Reference Parnmeterg tirrenia 73 Constant Paramete s s sscssssssesssssceesesscessonsssseenssssensssnsensenscesssnecesscnseesenseseseeenseesss 74 Open Array Pal aMeLerSs weisse E E EE 75 Type Variant Open Array Parameters iii 76 Delphi Calling Conventions insonini innin enne RCONT ehen 78 What EE KEE 79 Forward Declarations rire 79 Procedural Types RE 81 Furiction Overloading cca ia ia 83 Default Paramete ars A EAEE NEEN ETN NEES 85 elt 87 Chapter 7 Handlng Stiings crisrirrirazenicevensane 89 Types Of SHINES 89 Traditional Pascal Strings eoe eet re tr eee AL i anti 90 Using ENEE 91 Looking at Strings in MeMOry iii 92 Delphi Strings and Windows PChars iii 94 Formatting Strings ceo e ene mi 96 nbl aria detente 98 Chapter 8 M mOEy es veve see eees seas tovt ve sea S EES ENEE ENEE E eee Vue se e e VUE 99 Global Memory iraniana nta 99 The Stack Memory ra rai aiar 100 The Heap Memory csscsssssssssessscssesscescenssessensesssenssnsensesscenssnscenssaseesscneessenecensenseeess 100 Dynamic Array sy CR 101 er ON 103 Chapter 9 Windows ProgrammMing sscccccs
19. If you want to make a few experiments with expressions there is nothing better than writing a simple program As for most demos of this book create a console application and use writeln function statements to display something on the console output screen At the end of the program it is 8 In Pascal parameters passed to a function or procedures are enclosed in parenthesis Some other languages notably Rebol and Ruby let you pass parameters simply by writing them after the function or procedure name Marco Cant Essential Pascal Chapter 2 Coding in Pascal 25 common to add a read1n function call so that the program will wait until you press the Enter key and not close immediately in which case you might not see the output Here is the complete program of the demo program EPExpressions program EPExpressions SAPPTYPE CONSOLE begin writeln 20 5 writeln 30 222 writeln 3 lt 30 writeln 12 10 readln end From now on I ll generally show only the relevant code skipping some of the details like the read1n call and the APPTYPE directive The source code examples available with the book will have complete and running programs though This is the output of the program 100 252 TRUE FALSE Operators and Precedence If you have ever written a program in your life you already know what an expression is as they constitute the base building blocks of any program ming language Here I ll hig
20. Signed Unsigned 8 bits ShortInt 128 to 127 Byte 0 to 255 16 bits Smal 1Int 32768 to 32767 word 0 to 65 535 16 32 l bits Integer Cardinal 32 bits LONgENt 2147 483 04510 Longword 0 to 4 294 967 295 2 147 483 647 929459075 64 bits Int64 9223372036854775808 to 9223372036854775807 As you can see these types correspond to different representations of num bers depending on the number of bits used to express the value and the presence or absence of a sign bit Signed values can be positive or negative but have a smaller range of values because one less bit is available for the value itself You can refer to the Range example discussed in the next sec tion for the actual range of values of each type The last group marked as 16 32 indicates values having a different repre sentation in the 16 bit and 32 bit versions of Delphi Integer and Cardinal are frequently used because they correspond to the native representation of numbers in the CPU I ll explain the benefits of using Integer and Cardinal later Integer Types In Delphi 2 and 3 the 32 bit unsigned numbers indicated by the Cardinal type were actually 31 bit values with a range up to 2 gigabytes Delphi 4 introduced a new unsigned numeric type Longword which uses a truly 32 bit value up to 4 gigabytes The Cardinal type is now an alias of the Longword type Longword permits 2GB more data to be addressed by an unsigned number as mentioned above Mo
21. a then writeln You pressed a Marco Cantu Essential Pascal 62 Chapter 5 Statements If you enter the a character lowercase A the program will show a simple message Otherwise nothing happens In a case like this it would probably be better to make this more explicit as with the following code snippet which uses an if then else statement if then else statement if achar 5 then writeln vou pressed b else writeln You pressed something else than b Notice that you cannot have a semicolon after the first statement and before the else keyword or the compiler will issue a syntax error The i f then else statement is in fact a single statement so you cannot place a semi colon in the middle of it An if statement can be quite complex The condition can be turned into a series of conditions using the and or and not Boolean operators or the if statement can nest a second if statement The last part of the IfTest example demonstrates these cases statement with a double condition checks for a lowercase char if achar gt a and achar lt z then writeln owercase compound if statement if aChar gt Char 32 then begin if achar gt 0 and achar lt 2 then writeln C a number else writeln C not a number end else writeln non printable char Look at the code carefully and run the program to see if you understand everything you can us
22. a string to a Windows API function For example to copy the name of the current Windows user into a PChar string using the API function GetUserName and then display it var Strl string nsize Cardinal 20 begin SetLength Strl nSize GetuserName PChar Str1 nSize writeln str1 end You can find this code in the StringAndPChar example Note that if you write this code without first allocating memory for the string with SetLength the program will probably crash If you are using a PChar to pass a value and not to receive one as in the code above the code is simpler because there is no need to define a temporary string and initialize it Having presented the nice picture now I want to focus on the pitfalls There are some problems that might arise when you convert a long string into a PChar Essentially the underlying problem is that after this conversion you become responsible for the string and its contents and Pascal won t help you 46 When you need to cast a WT dest ring to a Windows compatible type you have to use Pwi deChar instead of PChar for the conversion Wide strings are often used for COM programs Marco Cant Essential Pascal Chapter 7 Handling Strings 95 any more Consider the following small change in the StringAndPChar exam ple with some text added to the output string GetUserName PChar Strl nSize writeln stri ei This program compiles but when you run it you a
23. based version and a printed book I ve kept the prices of the PDF and print version very low to suit an audience of students and hob byists although the book can serve professional developers as well Pascal as in Delphi This is a detailed book on the Pascal language found in Delphi and in some of the available Delphi dialects namely FreePascal and Chrome The book 1 There is also a GNU Pascal compiler which supports the ISO 7185 and ISO 10206 standards but it is really not commonly used by the Pascal community which is generally tied to non standard Borland extensions of the language Marco Cantu Essential Pascal 4 Introduction purposely focuses on the traditional Pascal language constructs not delving into its object oriented extensions From time to time there will be updates to cover extensions to the core language provided by recent editions of Del phi again not the OOP extensions of Object Pascal The first complete version of this book dated July 99 was published on the Delphi 5 Companion CD Following editions were updated both in the con tent and in the format with notes covering Kylix Delphi for Linux and Delphi for NET This new edition the first available in print extends cover age to other Pascal Delphi dialects During the various editions the examples used in the book were all migrated away from using Delphi s Visual Control Language VCL graphic user inter face GUI to make the boo
24. class is not enforced within the scope of the unit containing the class Marco Cantu Essential Pascal Chapter 11 Program and Units 123 form classes are declared in the same way so that you can refer to a form and its public fields methods properties and components from the code of any other form Of course it s poor programming practice to declare everything as global Besides the obvious memory consumption problems using global variables makes a program harder to maintain and update In short you should use the smallest possible number of global variables Units as Namespaces The uses statement is the standard technique to access the scope of another unit At that point you can access the definitions of the unit But it might happen that two units you refer to declare the same identifier that is you might have two classes or two routines with the same name In this case you can simply use the unit name to prefix the name of the type or routine defined in the unit For example you can refer to the ComputeTotal procedure defined in the given Totals unit as Totals ComputeTotal This should not be required very often as you are strongly advised against using the same name for two different things in a program However if you look into the VCL library and the Windows files you ll find that some Delphi functions have the same name as but generally different parameters than some Windows API functions available in Delphi i
25. date value Retrieves the hours minutes seconds and millisec onds from a date value Turns year month and day values into a date time value Turns hour minute second and millisecond values into a date time value To show you how to use this data type and some of its related routines I ve built a simple example named TimeNow When the program starts it auto matically computes and displays the current time and date Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 43 StartTime Now writeln TimeToStr StartTime writeln DateToStr StartTime The first statement is a call to the Now function which returns the current date and time This value is stored in the StartTime variable declared as a global variable as follows var StartTime TDateTime The next two statements display the time portion of the TDateTime value converted into a string and the date portion of the same value This is the output of the program 6 33 14 PM 10 7 2007 To compile this program you need to refer to functions that are part of a commonly used Pascal unit a source code file called SysUtils To accom plish this you have to write uses SysUtils Besides calling TimeToStr and DateToStr you can use the more powerful FormatDateTime function as I ve done in the last method above see the Help file or documentation for details on the formatting parameters Notice that time and date values are transfo
26. dynamically In 43 You can write this also with the classic notation strShortName string 20 44 The string location holds the data itself and not a pointer to the actual data as for long strings Marco Cant Essential Pascal Chapter 7 Handling Strings 91 this specific case you ll end up using 21 bytes 20 for the characters and one for the length This is traditionally called the length byte and is stored at the beginning of the string data Notice in fact that the string has a maxi mum size of 20 characters but during its lifetime the length of the string it stores might vary Given the assignment above the actual length will be five and it will consume 6 bytes By declaring their dimension upfront you are limited to a specific maximum size and cannot exceed it for any reason Moreover the maximum size you can declare is 255 simply because the length byte is a byte so it can only represent a number ranging from 0 to 255 On the other hand short strings are very fast as there is no dynamic alloca tion cleanup and no reference counting involved With a fixed size they can easily be stored in records and other data structures Indeed a fixed string size is a limitation but one that a database developer for example will easily live with Short strings are not heavily used in Pascal these days although they are cer tainly a key data structure of traditional Pascal That s why in this chapter I prefe
27. it When you add a new form to a project with the corresponding toolbar button or the File gt New Form menu command Delphi actually adds a new unit which defines the class for the new form Units Although every form is defined in a unit the reverse is not true Units do not need to define forms they can simply define and make available a collection of routines By selecting the File gt New menu command and then the Unit icon in the New page of the Object Repository you add a new blank unit to Marco Cantu Essential Pascal 120 Chapter 11 Program and Units the current project This blank unit contains the following code delimiting the sections a unit is divided into unit Unitl interface implementation end The concept of a unit is simple A unit has a unique name corresponding to its filename an interface section declaring what is visible to other units and an implementation section with the real code and other hidden decla rations Finally the unit can have an optional initialization section with some startup code to be executed when the program is loaded into memory it can also have an optional finalization section to be executed on pro gram termination The general structure of a unit with all its possible sections is the following unit unitName interface other units we refer to in the interface section uses A B Cj exported type definitions type newType TypeDefinition expo
28. merge two sets Array Types Array types define lists of a fixed number of elements of a specific type You generally use an index within square brackets to access one of the elements of the array Square brackets are also used to specify the possible values of the index when the array is declared For example you can define a group of 24 integers with this code type TDayTemperatures array 1 24 of Integer 23 When working with the Object Inspector in Delphi you can provide the values of a set by expanding the selection double click on the property name or click on the plus sign on its left and toggling on and off the presence of each value Marco Cant Essential Pascal Chapter 4 User Defined Data Types 53 In the array definition you need to pass a subrange type within square brackets or define a new specific subrange type using two constants of an ordinal type This subrange specifies the valid indexes of the array Since you specify both the upper and the lower index of the array the indexes don t need to be zero based as is necessary in C C Java and other languages Since the array indexes are based on subranges Pascal can check their range as we ve already seen An invalid constant subrange results in a compile time error and an out of range index used at run time results in a run time error if the corresponding compiler option is enabled Using the array definition above you can set the value of a Da
29. one of a different type When you need to do this there are two choices The first choice is typecasting which uses a simple functional notation with the name of the destination data type var N Integer C Char B Boolean begin N Integer x C char N B Boolean 0 You can typecast between data types having the same size It is usually safe to typecast between ordinal types or between real types but you can also typecast between pointer types and also objects as long as you know what you are doing Marco Cantu Essential Pascal Chapter 3 Types Variables and Constants 45 Casting however is generally a dangerous programming practice because it allows you to access a value as if it represented something else Since the internal representations of data types generally do not match you risk acci dentally creating hard to track errors For this reason you should generally avoid typecasting The second choice is to use a type conversion routine The routines for the various types of conversions are summarized in the following list chr ord Round Trunc Int IntToStr IntToHex StrToInt StrToIntDef Val str StrPas StrPCopy Converts an ordinal number into an ANSI character Converts an ordinal type value into the number indi cating its order Converts a real type value into an Integer type value rounding its value Converts a real type value into an Integer type va
30. out to adopt the NET architecture Marco Cantu Essential Pascal 112 Chapter 9 Windows Programming Marco Cantu Essential Pascal Chapter 10 Variants 113 Chapter 10 Variants To provide full Windows OLE support the 32 bit version of Delphi includes the variant data type Here I want to discuss this data type from a general perspective The Variant type in fact has a pervasive effect on the whole language and the Delphi components library also uses them in ways not related to OLE programming Variants Have No Type In general you can use variants to store any data type and perform numer ous operations and type conversions Notice that this goes against the general approach of the Pascal language and is an implementation of a kind of dynamic typing such as employed in Smalltalk Objective C and may pop ular scripting languages including JavaScript PHP Python and Ruby A 50 FreePascal mimics Delphi s variant type 51 I demonstrated this dynamic language extension approach in a a talk I gave for the first time at the CodeRage II virtual conference in November 2007 covering Domain Specific Languages in Delphi Marco Cant Essential Pascal 114 Chapter 10 Variants variant is type checked and computed at run time The compiler won t warn you of possible errors in the code which can be caught only with extensive testing On the whole you can consider the code portions that use variants to be interp
31. overall structure of Delphi programs Also units have influence on scope and global memory allocations 53 For more information see http www modula2 org modula 2 php and http en wikipedia org wiki Modula 2 Marco Cantu Essential Pascal 126 Chapter 11 Program and Units Marco Cantu Essential Pascal Chapter 12 Files in the Pascal Language 127 Chapter 12 Files In The Pascal Language One of the unique aspects of Pascal compared to other programming lan guages is its built in support for files The language has a file keyword which is a type specifier like array or record You use file to define a new type and then you can use the new data type to declare new variables type IntFile file of Integers var IntFilel IntFile It is also possible to use the file keyword without indicating a data type to specify an untyped file Alternatively you can use the TextFi le type defined in the System units to declare files of ASCII characters Each kind of file has its own predefined routines as we will see later in this chapter 54 Notice that the file of language construct doesn t work in Delphi for NET as it is bound to the physical size of the date types it manages Marco Cantu Essential Pascal 128 Chapter 12 Files in the Pascal Language Routines for Working with Files Once you have declared a file variable you can assign it to an actual file in the file system using the AssignFile method The next
32. parameters Ist StrParam Parameter MB OK end The output code uses the MessageBox API function simply to avoid includ ing the entire VCL into the project A pure Windows program as the one above in fact has the advantage of a very small memory footprint The exe cutable file of the program is about 18 KB Marco Cantu Essential Pascal Chapter 9 Windows Programming 111 To provide a command line parameter to this program you can use Delphi s Run gt Parameters menu command Another technique is to open the Win dows Explorer locate the directory that contains the executable file of the program and drag the file you want to run onto the executable file The Win dows Explorer will start the program using the name of the dropped file as a command line parameter Summary In this chapter we ve seen an introduction to low level Windows program ming discussed handles and a very simple Windows program For normal Windows programming tasks you ll generally use the visual development support provided by Delphi based on the VCL But that is beyond the scope of this book which is the Pascal language Next chapter covers variants a very strange addition to the Pascal type sys tem originally introduced to provide full Windows OLE support 49 OLE and COM are far too complex to be covered in this text For some years they were core Windows technologies and they are still heavily used although Microsoft is phasing them
33. string examples below suggestions omitted readln strFmt writeln Format d Se gt Ss In strFmt Format strFmt n end The code basically does the formatting operation using the second input as format string and the value of the first input request for the value The float ing point number formatting is similar Summary Strings are certainly a very common data type Although you can safely use them in most cases without understanding how they work this chapter should have made the exact behavior of strings clear making it possible for you to use all the power of this data type Strings are handled in memory in a special dynamic way as happens with dynamic arrays This is the topic of the next chapter Marco Cantu Essential Pascal Chapter 8 Memory 99 Chapter 8 Memory This chapter covers memory handling the various memory areas you can use in an application and introduces dynamic arrays Global Memory The global memory is the memory area for global variables As you declare a global Integer variable for example the compiler will reserve 4 bytes of global memory for it Regardless of the visibility of this global data see Chapter 11 for units and their related visibility restrictions it allocates global memory Global variables are allocated when the program starts in a specific global data memory whose size and layout are defined by the compiler and linker Global variables remain i
34. to an include directive The effect of a uses statement is to import just the pre compiled interface portion of the units listed The implementation portion of the unit is considered only when that unit is compiled The units you refer to can be both in source code format PAS or compiled format DCU with Delphi but the compilation must have taken place with the same version of the Pascal compiler The interface of a unit can declare a number of different elements includ ing procedures functions global variables and data types In Delphi applications data types are probably used the most Delphi automatically places a new class data type in a unit each time you create a form However containing form definitions is certainly not the only use for units in Delphi Marco Cant Essential Pascal 122 Chapter 11 Program and Units You can continue to have traditional units with functions and procedures and you can have units with classes that do not refer to forms or other visual elements Units and Scope In Pascal units are the key to encapsulation and visibility and they are probably even more important than the private and public keywords of a class The scope of an identifier such as a variable procedure function or a data type is the portion of the code in which the identifier is accessible The basic rule is that an identifier is meaningful only within its scope that is only within the block in which it is declared
35. 24 DWORD i Emden eiae ewe 44 Marco Cant Essential Pascal 138 Index Dynamic Arrans iii 101 Dynamic link library 107 ElS6 e eere e E e tein A En 62 EncodeDate 42 LT eeh Mle Monat 70 Enumerated rin 50 EnumWindows iii 108 HOE EE 132 Example 5 nire tne terere Case L ae erre rere e rere EE HERE d 63 DoubleH 5 siete 80 DYDATT iier rte 102 EPConstants eere 31p EPExpressions eene 25 EPR ng6 5 oer 36 40 Filter i ar RE 131 Pm Tasta iii 97 Por Tog asar rere E Oar a 65 IK EE 62 IntegersToFile 128 EOODSTeSL idet eerta 66 15T eier 76 78 OVverDef itato 83 85 POINters iii 57 Proc Types esenee eser lancia 82 Range eerte ina 34 StringAndPChar i 94p StringsToFile esses 130 jud M RATE DE 92 TAmelNOW eet oes too eroe ne eios exo 42 Variable 8 iiiter tii meten 30 VarlTest iz cse ies 114 MAIN 116 EXIU A este ehe rn 67 Extended ences 39 External declarations 107 Eastealls tober etnies 78 Feedback nicer eer redeo nee 5 File ueteres sabe tees ness 58 127 FileSize reir e 132 Finalization in 120 FloatToDecimal eese 46 aR tti 46 BOR OS 64 Der a EE 65 Marco Canto Essential Pascal Formati riore 96 FormatDateTime 42
36. C language To overcome the limits of traditional Pascal strings modern versions of Pas cal support long strings There are actually three string types e The ShortString type corresponds to the original Pascal strings as described before These strings have a limit of 255 characters and correspond to the strings in the 16 bit version of Delphi Each ele ment of a short string is oftype ANSIChar the standard character type e The ANSIString type corresponds to the new variable length long strings These strings are allocated dynamically are reference counted which means that you don t need to worry about releasing the memory that they use and use a copy on write technique The size of these strings is almost unlimited they can store up to two bil lion characters They are also based on the ANSIChar type e ThewideString type is similar to the ANSIString type but is based on the wi deChar type it stores Unicode characters It is not as powerful and efficient as the standard string types as its support for reference counting is not as complete Traditional Pascal Strings Traditional Pascal strings are a very simple and effective data structure You can declare a string holding a maximum of 20 characters by declaring var strShortName Shortstring 20 begin strshortName marco The data will be allocated locally on the stack the memory area used by procedures and functions for their local variables and not
37. Cant Essential Pascal Table Contents 7 Table Contents Introd ction nasa cera zeri NEN NE Va tera E NE REN s 3 Pascal Ee E 3 Book Copyright and Availability ii 4 Ke 5 Feedback aaa 5 ee 5 About theAutho iagrer accaniti 6 Chapter 1 Short History of the Pascal Language scccccscsscsscsccscssssceccscsscscscescsssces 11 Warth s PasCallis MR E 12 KN le 12 Delphi s Pascal EE 13 Chapter 2 Coding in Pascal eege Ero SEENEN eve e eR ge ee E 15 e UE 16 COMMENES iii 16 Use Of Uppercase e 17 Mure 18 Pretty Printin EE 19 Syntax Highlightitg rere deg E RIO rt 20 Error Insight and Coding Helper 21 Language Statement renin a n T ioni 21 Keyword So aiar ira or 22 lateral Values iaia eo en 23 Expressions and Operators iii 24 Marco Cantu Essential Pascal 8 Table Contents Showing the Result of an Expression a First Program 24 Operators and Precedence scsrsrrrcrererecescezeeee risco reso nie sensore vene Ea 25 e EE 27 Conclusioni x Faria ME Eet ee een d der 27 Chapter 3 Types Variables and Constants cccscoscsscsccscsscssceccscssccscscescscecsscscecess 29 Variables teet erm iram Rino 30 LS 21T E TTD UT PLE 31 Resource String ConSLants se ere etre ro era oett re deae 32 Data K D ll IL IA asl nadal 33 Ordinal TyPes ri siria nare iaia 33 Integer Types o
38. Essential Pascal Chapter 6 Procedures and Functions 73 changing the code of this procedure and the main program code will auto matically change its effect Here is how we can change the code procedure Hello begin writeln He o world again end When you call an existing Pascal function or procedure you need to remem ber the number and type of the parameters Reference Parameters Pascal routines allow parameter passing by value and by reference Passing parameters by value is the default the value is copied on the stack and the routine uses and manipulates the copy not the original value Passing a parameter by reference means that its value is not copied onto the stack in the formal parameter of the routine avoiding a copy often means that the program executes faster Instead the program refers to the original value also in the code of the routine This allows the procedure or function to change the value of the parameter Parameter passing by reference is expressed by the var keyword This technique is available in most programming languages It isn t present in C but has been introduced in C where you use the amp pass by refer ence symbol In Visual Basic every parameter not specified as ByVal is passed by reference Here is an example of passing a parameter by reference using the var keyword procedure DoubleThevalue var Value Integer begin Value Value 2 end In this case the paramete
39. Incidentally there is no run time overhead for using Low and Hi gh with arrays They are resolved at compile time into constant expressions not actual function calls This compile time resolution of expressions and function calls also happens for many other simple system functions Marco Cant Essential Pascal Chapter 4 User Defined Data Types 55 type TMyDate record Year Integer Month Byte Day Byte end var BirthDay TMyDate begin BirthDay Year 1997 BirthDay Month 2 BirthDay Day 14 end Classes and objects can be considered an extension of the record type Del phi libraries tend to use class types instead of record types but there are many record types defined by the Windows API Record types can also have a variant part that is multiple fields can be mapped to the same memory area even if they have a different data type This corresponds to a union in the C language Alternatively you can use these variant fields or groups of fields to access the same memory location within a record but considering those values from different perspectives The main uses of this type were to store similar but different data and to obtain an effect similar to that of typecasting something used in the early days of Pascal when direct typecasting was not allowed The use of variant record types has been largely replaced by object oriented and other modern techniques although Delphi uses them internally in som
40. Marco Cant Essential Pascal Piacenza Italy 4 Edition April 2008 Author and publisher Marco Cant Editor Peter W A Wood Tech Editors for this edition Patricio Moschcovich Keld R Hansen Cover Designer Fabrizio Schiavi Copyright 1995 2008 Marco Cant Piacenza Italy World rights reserved The author created example code in this publication expressly for the free use by its read ers The source code for this book is copyrighted freeware distributed via the web site http ww marcocantu com The copyright prevents you from republishing the code in print media without permission Readers are granted limited permission to use this code in their applications as long at the code itself is not distributed sold or com mercially exploited as a stand alone product Permission to add limited pieces of code to your application is fine regardless of the application s license Aside from this specific exception concerning source code no part of this publication may be stored in a retrieval system transmitted or reproduced in any way in the original or in a translated language including but not limited to photocopy photograph magnetic or other record without the prior agreement and written permission of the publisher ISBN 1440480117 EAN 13 9781440480119 Delphi is a trademark of CodeGear a subsidiary of Borland Windows and Windows Vista are trademarks of Microsoft Other trademarks are of the respective owner
41. P OO end end When the user presses any other key the else branch of the case above the procedure we have stored is executed You can see a more practical example of the use of procedural types in Chapter 9 in the section A Win dows Callback Function Function Overloading The idea of overloading is simple The compiler allows you to define two functions or procedures using the same name provided that the parameters are different By checking the parameters in fact the compiler can deter mine which of the versions of the routine you want to call Consider this series of functions extracted from the Math unit of the VCL function Min A B Integer Integer overload function Min A B Int64 Int64 overload function Min A B Single Single overload function Min A B Double Double overload function Min A B Extended Extended overload When you call Min 10 20 the compiler easily determines that you re calling the first function of the group so the return value will be an Integer The basic rules are two e Each version of the routine must be followed by the overload key word e The differences must be in the number or type of the parameters or both The return type cannot be used to distinguish among two rou tines Here are three overloaded versions of a ShowMsg procedure I ve added to the OverDef example an application demonstrating overloading and default parameters Marco Cantu Essential P
42. acter 255 is a white space The size of the Integer type varies depending on the CPU and operating system you are using In 16 bit Windows that is using Delphi 1 for example an Integer variable is two bytes wide In 32 bit Windows including all later versions of Delphi until now an Integer is four bytes wide The different representations of the Integer type are not a problem as long as your program doesn t make any assumptions about the size of integers If you happen to save an Integer to a file using one version and retrieve it with another though you re going to have some trouble In this situation 13 Delphi has a rich support for Run Time Type Information RTTI you can use to operated on directly on data types at runtime but this topic as you can probably guess is quite advanced and way outside of the scope of this book Marco Cantu Essential Pascal 38 Chapter 3 Types Variables and Constants you should choose a platform independent data type such as LongInt Smallint or Int64 For mathematical computation or generic code your best bet is to stick with the standard integer representation for the specific platform that is use the Integer type because this is what the CPU likes best The Integer type should be your first choice when handling integer num bers Use a different representation only when there is a compelling reason to do so Ordinal Types Routines There are some system routines routin
43. alue of a property is an enumeration you can choose its value from the list displayed in the Object Inspector Marco Cant Essential Pascal 52 Chapter 4 User Defined Data Types You can revise the operations you can make on a set which comprise the Include and Exclude statements in Set Operators section of Chapter 2 In Pascal a set is generally used to indicate nonexclusive flags For example the following two lines of code which are part of the Delphi library declare an enumeration of possible icons for the border of a window and the corre sponding set type type TBorderIcon bisystemMenu biMinimize biMaximize biHelp TBorderIcons set of TBordericon In fact a given window might have none of these icons one of them or more than one Another property based on a set type is the style of a font Possible values indicate a bold italic underline and strike through font Of course the same font can be both italic and bold have no attributes or have them all For this reason it is declared as a set You can assign values to this set in the code of a program as follows Font Style no style Font Style fsBold bold style only Font Style fsBold fsItalic two styles You can also operate on a set in many different ways including adding two variables of the same set type or to be more precise computing the union of the two set variables Font Style oldstyle fsunderline
44. an be used directly to define a variable without an explicit type name as in the following code var DecemberTemperature array 1 31 of Byte ColorCode array Red violet of word Palette set of Colors In general you should avoid using unnamed types as in the code above because you cannot pass them as parameters to routines or declare other variables of the same type The type compatibility rules of Pascal in fact are based on type names not on the actual definition of the types Two variables of two identical types are still not compatible unless their types have exactly the same name and unnamed types are given internal names by the compiler Get used to defin Marco Cant Essential Pascal Chapter 4 User Defined Data Types 49 ing a data type each time you need a variable with a complicated structure and you won t regret the time you ve spent doing it But what do these type definitions mean UU provide some descriptions for those who are not familiar with Pascal type constructs I ll also try to under line the differences from the same constructs in other programming languages so you might be interested in reading the following sections even if you are familiar with the kind of type definitions listed above Finally I ll show some examples and introduce some tools that will allow you to access type information dynamically Subrange Types A subrange type defines a range of values within the range of another ty
45. and one program file The units can be considered secondary files which are referred to by the main part of the application the program In theory this is true In practice the program file is usually an automatically generated file with a limited role It simply needs to start up the program running the main form The code of the program file or Delphi project file DPR can be edited either manually or by using the Project Manager and some of the Project Options related to the application object and the forms The structure of the program file is usually much simpler than the structure of the units Here is the source code of a sample program file program Projectl uses Forms Unitl in uniti PAS Formi begin Application Initialize Application CreateForm TForml Forml Application Run end Marco Canto Essential Pascal Chapter 11 Program and Units 125 As you can see there is simply a uses section and the main code of the appli cation enclosed by the begin and end keywords The program s uses statement is particularly important because it is used to manage the compi lation and linking of the application Summary Units were the Pascal actually Turbo Pascal as Wirth added the concept in the Modula 2 language technique for modular programming Even if they were later followed by objects and classes they still play a key role for encap sulation for the definition of some sort of name space and for the
46. ariant see for example the VarType function Most of these type conversion and assignment functions are actually called automati cally when you write expressions using variants Other variant support Marco Cantu Essential Pascal 116 Chapter 10 Variants routines look for the topic Variant support routines in the Help file actu ally operate on variant arrays Variants Are Slow Code that uses the Variant type is slow not only when you convert data types but also when you add two variant values holding integers They are almost as slow as the interpreted code of Visual Basic To compare the speed of an algorithm based on variants with that of the same code based on inte gers you can look at the VSpeed example This program runs a loop timing its speed and showing the status in a progress bar Here is the first ofthe two very similar loops based on integers and variants var timel time2 TDateTime begin timel Now nl 0 n2 0 while n1 5000000 do begin n2 n2 nl Inc n1 end we must use the result writeln n2 time2 Now writeln varzants FormatDateTime ss hhh Time2 Timel seconds D The timing code is worth looking at because it s something you can easily adapt to any kind of performance test As you can see the program uses the Now function to get the current time and the FormatDateTime function to output the time difference asking only for the seconds
47. ascal 84 Chapter 6 Procedures and Functions procedure ShowMsg str string overload begin writeln Message end str procedure ShowMsg FormatStr string Params array of const overload begin writeln Message end Format FormatStr Params procedure ShowMsg I Integer Str string overload begin ShowMsg IntToStr I Str end The three functions show a message box with a string after optionally for matting the string in different ways Here are the three calls of the program ShowMsg He o ShowMsg 7ota d 100 ShowMsg 10 Bytes And this is their effect Message Hello Message Total 100 Message 10 MBytes The fact that each version of an overloaded routine must be properly marked implies that you cannot overload an existing routine of the same unit that is not marked with the overload keyword The error message you get when you try is Previous declaration of lt name gt was not marked with the overload directive However you can overload a routine that was originally declared in a differ ent unit This is for compatibility with previous versions of Delphi which allowed different units to reuse the same routine name Notice anyway that this special case is not really an extra feature of overloading as it seems to have little effect 41 Delphi IDE s Code Parameters technology works very nicely with overloaded procedures and funct
48. b to Spade do All these code fragments are part of the ForTest example The last loop can also be written to explicitly operate on each element of the enumeration by writing for aSuit Low TSuit to High TSuit do Recent versions of Delphi introduce a new form of loop called for in which resembles the traditional Visual Basic for each loop In this kind of for loop the cycle operates on each element of an array a list or some other form of container While and Repeat Statements The difference between the whi 1e do loop and the repeat unti1 loop is that the code of the repeat statement is always executed at least once You can easily understand why by looking at a simple example while I lt 100 and J lt 100 do begin use I and J to compute something I I 1 Jo je Bods end repeat use I and J to compute something I 1 l Jie op until I gt 100 or J gt 100 If the initial value of I or J is greater than 100 the statements inside the repeat unti loop are executed once anyway The other key difference between these two loops is that the repeat unti 1 loop has a reversed condition This loop is executed as long as the condition is not met When the condition is met the loop terminates This is the oppo site of a whi 1e do loop which is executed while the condition is true For 33 The details of the for in loop are covered in my Delphi 2007 Handbook Marco Cantu Es
49. before Pascal has some specific support for text files most notably the TextFile data type defined by the System unit In the StringsToFile example I create a file the filename must be passed as parameter with some textual content var OutputFile TextFile I Integer Filename string begin filename ParamStr 1 if filename then begin writeln Missing file name end else begin output the text to a file AssignFile outputFile FileName Rewrite OutputFile for I 1 to 10 do writeln OutputFile ztem IntToStr 1 CloseFile outputFile writeln done end readln end Instead of being connected to a physical file Pascal text files can be hooked directly to the printer so that the output will be printed instead of being saved to a file To accomplish this simply use the AssignPrn procedure For example in the code above you could replace the line AssignFile outputFile FileName with the line AssignPrn OutputFi le Marco Cantu Essential Pascal Chapter 12 Files in the Pascal Language 131 A Text File Converter Up to now we ve seen simple examples of creating new files In our next example we ll process an existing file creating a new one with a modified version of the contents The program named Filter can convert all the char acters in a text file to uppercase capitalize only the initial word of each sentence or ignore the characters from the upper portion o
50. but do not know how many elements of that type the array is going to have Here is an example of such a definition function Sum const A array of Integer Integer var I Integer begin Result 0 for I Low A to High A do Result Result A I end Using High A we can get the upper bound of the array Notice also the use of the return value of the function Result to store temporary values You can call this function by passing to it an array of Integer expression X Sum 10 v 27 I Given an array of Integer of any size you can pass it directly to a rou tine requiring an open array parameter or instead you can call the Slice function to pass only a portion of the array as indicated by its second parameter Here is an example where the complete array is passed as parameter var List array 1 10 of Integer X I Integer begin initialize the array for I Low List to High List do List I I 2 call X Sum List Marco Cantu Essential Pascal 76 Chapter 6 Procedures and Functions If you want to pass only a portion of the array to the Sum function simply call it this way using the standard S1i ce function X Sum Slice List 5 Did you notice how the parameter to one function can be the result of another You can find all the code fragments presented in this section in the OpenArr example Typed open arrays are fully compatible with dynamic arrays introduc
51. call to IOResult resets it value which is why we need to call it once and keep the result value around to report it by calling IOResult again for the reporting code will get a o result meaning no error Delphi includes many other file management routines some of which are in the list below Append Fileclose Flush AssignFile Fi lecreate GetDir BlockRead FileDateToDateTime IOResult Blockwrite FileExists MkDir ChangeFileExt FileGetAttr Read CloseFile FileGetDate Readln DateTimeToFileDate Fileopen Rename DeleteFile FilePos RenameFile DiskFree Fi leRead Reset Disksize FileSearch Rewrite Eof FileSeek RmDir Eoln FileSetAttr Seek Erase Fi leSetDate SeekEof ExpandFi leName FileSize SeekEoln ExtractFileExt Filewrite SetTextBuf ExtractFileName FindClose Truncate ExtractFilePath FindFirst write Fi leAge FindNext writeln Not all of these routines are defined in standard Pascal but many of them have been part of Turbo Pascal since the early days You can find detailed information about these routines in Delphi s Help files Here I ll show you two simple examples based on text files to demonstrate how some of these features can be used The second example will include command line pro cessing and is possibly the most complete example of the entire book Marco Cantu Essential Pascal 130 Chapter 12 Files in the Pascal Language Handling Text Files One of the most commonly used file formats is that of text files As I men tioned
52. case Let s consider the Enumwi ndows API function which has the following pro totype in the Pascal language definition function Enumwindows lpEnumFunc TFNWndEnumProc lParam LPARAM BOOL stdcall Consulting the help file we find that the function passed as a parameter should be of the following type again in the Pascal version type EnumwindowsProc function Hwnd THandle Param Pointer Boolean stdcall The first parameter is the handle of each main window in turn while the sec ond is the value we ve passed when calling the Enumwindows function Actually in Pascal the TFNWndEnumProc type is not properly defined it is simply a pointer This means we need to provide a function with the proper parameters and then use it as a pointer taking the address of the function instead of calling it Unfortunately this also means that the compiler will provide no help in case of an error in the type of one of the parameters Windows requires us programmers to follow the stdca11 calling convention every time we call a Windows API function or pass a callback function to the system Delphi by default uses a different and more efficient calling con vention indicated by the register keyword Here is the definition of a proper compatible function which reads the title of the window into a string then lists it on screen function GetTitle Hwnd THandle Param Pointer Boolean stdcall Marco Canto Essential Pascal Chap
53. ce var or con stant const parameter doesn t imply a further reference to the string In this case I ve used a const parameter as the function is not supposed to modify the string To obtain the memory address of the string useful to determine its actual identity and to see when two different strings refer to the same memory area I ve simply made a hard coded typecast from the string type to the Integer type Long strings are references in practice they re pointers Their value holds the actual memory location of the string not the string itself To extract the reference count I ve based the code on the little known fact that the length and reference count are actually stored in the string before the actual text and before the position the string variable points to The neg ative offset is 4 for the length of the string a value you can extract more easily using the Length function and 8 for the reference count By running this example you should get two strings with the same content the same memory location and a reference count of 2 Strl Address 13419088 Length 5 References 2 Value Hello Str2 Address 13419088 Length 5 References 2 Value Hello Now if you change the value of one of the two strings it doesn t matter which one the memory location of the updated string will change This is the effect of the copy on write technique We can actually produce this effect by writing the following code stri 2
54. coscsscsccscescsccsccscsccscsscescescessscescessscscesces 105 Windows Handles arri 106 External Declarations sce snai 107 Marco Cantu Essential Pascal 10 Table Contents A Windows Callback Function iii 108 A Minimal Windows Program 110 el EE 111 Chapter 10 Variants OTO IO II III 113 Variants Have NO Type re iicet te emere e ey e SANTINI iii 113 Variants in Depth Hie e e ree tane dE EE ee 115 VariantSAreSIow iar eran 116 intl d EE 117 Chapter 11 Program and Unils eee vnga ziaes eaa Suh oo Aa y aee rues RRV Eae nna cectesevevasecenseece 119 LR EC 119 Units and ee a 122 Units as Eet 123 Units and Programs dh erret aa rere eee yr ek ri 124 er O 125 Chapter 12 Files in the Pascal Language c cccccsscscsscsscsccscsscsscsscscsscsscsscescssscescsceces 127 Routines for Working with les 128 Handling Text Files AEN 130 A Text File Converter nihil 131 eru E 133 POSUB ACC oon ieceeciscks vcccetiscccescdecescecdestecstecvecetcesSeeeesastecseccessceeseedecesewesesesseasees 135 Appendix EXAM POS et 136 TOON PPS PICASA ORA POOR VORRAI SI PEARCE RENE bees bees KEES SSES 137 Marco Canto Essential Pascal Chapter 1 Short History of the Pascal Language 11 Chapter 1 Short History Of The Pascal Language The Object Pascal programming language we use in Delphi wasn t invented in 1995 along with the Borland visual development environment It was s
55. covers an extension to the core Pascal language the Variant data type Marco Cantu Essential Pascal 104 Chapter 8 Memory Marco Cantu Essential Pascal Chapter 9 Windows Programming 105 Chapter 9 Windows Programming Delphi provides complete encapsulation of the low level Windows API using Object Pascal and the Visual Component Library VCL It is rarely neces sary to build Windows applications using plain Pascal and calling Windows API functions directly Nevertheless programmers who want to use some special techniques not supported by the VCL or who don t want to use a visual environment still have that option As this book focuses on Pascal I ll show you how you could build a Pascal Windows application an excuse for learning some of the core features of the operating system something useful even when using a high level frame work You can build Pascal Linux and Mac OS X applications in a similar fashion by accessing a different API such as GTK 2 under Linux and Carbon for Mac OSX The principles are the same but I m going to concentrate on Windows in this chapter 47 FreePascal does something similar with Lazarus Component Library LCL which is on many counts a sort of a clone of the VCL Marco Cantu Essential Pascal 106 Chapter 9 Windows Programming Similarly the principal of accessing the Windows API from FreePascal or GNU Pascal is the same as accessing it from Delphi a few of the details may var
56. ction or procedure or termi nate the program The With Statement The last kind of Pascal statement I ll focus on is the wi th statement which used to be peculiar to this programming language although it has since been introduced in JavaScript and Visual Basic and can be very useful in Pascal programming 35 I find it preferable to use standard conditional statements if rather than Break and Continue as the code will generally be more clear Marco Cantu Essential Pascal 68 Chapter 5 Statements The wi th statement is nothing but a shorthand When you need to refer to a record type variable or an object instead of repeating its name every time you can use a wi th statement For example while presenting the record type I wrote this code type TMyDate record Year Integer Month Byte Day Byte end var BirthDay TMyDate begin BirthDay Year 2007 BirthDay Month 2 BirthDay Day 14 Using a with statement I can improve the final part of this code as follows with BirthDay do begin Year 2008 Month 2 Day 14 end This approach can be used in Pascal programs to refer to components and other class types When you work with components or classes in general the with statement allows you to skip writing some code particularly for nested fields For example in Delphi VCL code suppose that you need to change the width and the Color of the drawing pen for a form You can write the
57. different ways X 10 PA 20 When you have a pointer P with the expression P you refer to the address of the memory location the pointer is referring to and with the expression PA you refer to the actual content of that memory location For this reason in the code fragment above PA corresponds to x Instead of referring to an existing memory location a pointer can refer to a new memory block dynamically allocated on the memory heap with the New procedure In this case when you don t need the value accessed by the pointer anymore you ll also have to get rid of the memory you ve dynami cally allocated by calling the Di spose procedure If you don t dispose of the memory after using it your program may eventu ally use up all the available memory and crash This is known as a Memory Leak 26 Memory management in general and the way the heap works in particular are covered in Chapter 8 In short the heap is a large area of memory in which you can allocate and release blocks of memory in no given order 27 Asan alternative to New and Dispose you can use GetMem and FreeMem According to the Delphi help however it is considered preferable to use the Newand Dispose procedures Marco Cant Essential Pascal Chapter 4 User Defined Data Types 57 Here is a code snippet var P AInteger begin initialization New P operations PA 20 writeln PA termination Dispose P end If a
58. e These placeholders provide a default output for the given data type However you can use further format specifiers to alter the default output A width specifier for example determines a fixed number of characters in the output while a precision specifier indicates the number of decimal digits For example Format 8d n1 converts the number n1 into an eight character string right aligning the text use the minus symbol to specify left justification filling it with white spaces Here is the list of formatting placeholders d decimal The corresponding integer value is converted to a string of decimal digits x hexadecimal The corresponding integer value is converted to a string of hexadecimal digits Marco Cantu Essential Pascal p pointer s string e exponential f floating point g general n number m money Chapter 7 Handling Strings 97 The corresponding pointer value is converted to a string expressed with hexadecimal digits The corresponding string character or PChar value is copied to the output string The corresponding floating point value is converted to a string based on scientific notation The corresponding floating point value is converted to a string based on floating point notation The corresponding floating point value is converted to the shortest possible decimal string using either floating point or exponential notation The corresponding floating poi
59. e Char type which from Delphi 3 to Delphi 2007 corresponds to ANSIChar Keep in mind anyway that the first 256 Unicode characters correspond exactly to the ANSI characters Constant characters can be represented with their symbolic notation as in k or with a numeric notation as in 78 The latter can also be expressed using the Chr function as in Chr 78 The opposite conversion can be done with the Ord function It is generally better to use the symbolic nota tion when indicating letters digits or symbols When referring to special characters like those below 32 you ll generally use the numeric notation The following list includes some of the most com monly used special characters 12 There are also functions which can convert strings to these 64 bit integers I ll cover moving data from strings to integers and vice versa later in the book Marco Cantu Essential Pascal 36 Chapter 3 Types Variables and Constants 8 9 10 13 27 Disp backspace tabulator newline carriage return escape laying Ordinal Ranges To give you an idea of the different ranges of some of the ordinal types I ve written a simple Delphi program named EPRange The EPRange program displays the name size and range of some data types separating the values belonging to the same data type with tabulators program EPRange 1 APPTYPE CONSOLE begin write Integer write 9 write sizeof Integer write
60. e Code Completion help you write code by providing a list of legal symbols in the place where you are writing However these are edi tor specific features that I don t want to delve into in detail as I want to remain focused on the language and not discuss the Delphi editor even if the Delphi editor is one of the most common tools used for writing Pascal code Language Statements Once you have defined some identifiers you can use them in statements and in the expressions that are part of some statements Pascal offers several statements and expressions Let s look at keywords expressions and opera tors first Marco Cantu Essential Pascal 22 Chapter 2 Coding in Pascal Keywords Keywords are all the identifiers reserved by the Pascal or Object Pascal lan guage These are symbols that have a predefined meaning and role Delphi s Help distinguishes between reserved words and directives Reserved words cannot be used as identifiers while directives should not be used as such even if the compiler will accept them In practice you should not use any keyword as an identifier The following is a complete list of the identifiers including keywords and other reserved words Some of them have multiple meanings some are com monly used other rather obscure Even if you are an experienced Delphi programmer you might find one or two you ve never heard about Look them up in the help absolute abstract and array as asm assemb
61. e other languages including all those derived from C like C Java and C the Pascal compiler ignores case the capitalization of characters 4 Since Delphi 2006 and Turbo Delphi you can comment or uncomment a line or a group of lines with a direct keystroke This is Ctrl on the US keyboard and a different combination with the physical key on other keyboards 5 Actually compiler directives are still comments For example X This is a comment is legal It s both a valid directive and a comment although any sane programmers will probably tend to separate directives and comments Marco Cantu Essential Pascal 18 Chapter 2 Coding in Pascal Therefore the identifiers Myname MyName myname myName and MYNAME are all exactly the same In my opinion case insensitivity is definitely a posi tive feature as syntax errors and other subtle mistakes can be caused by incorrect capitalization in case sensitive languages There are a couple of subtle drawbacks however First you must be aware that these identifiers really are the same so you must avoid using them as different elements Second you should try to be consistent in the use of uppercase letters to improve the readability of the code A consistent use of case isn t enforced by the compiler but it is a good habit to get into A common approach is to capitalize only the first letter of each identifier When an identifier is made up of several consecutive words yo
62. e special cases The use of a variant record type is not type safe and is not recommended programming practice particularly for beginners Expert programmers can indeed use variant record types and the core of the Delphi libraries makes use of them You won t need to tackle them until you are really a Delphi expert anyway Before using variant record types consider also that the NET version of Del phi doesn t fully support them as they can be used only in unsafe portions of code These features in fact are not considered safe by the NET runtime and for a good reason 25 In recent versions of Delphi records can also have methods that is functions associated with them and support the overloading of language operators These topics are covered in my Delphi 2007 Handbook see my web site for details Marco Cantu Essential Pascal 56 Chapter 4 User Defined Data Types Pointers A pointer type defines a variable that holds the memory address of another variable of a given data type or an undefined type So a pointer variable indirectly refers to a value The definition of a pointer type is not based on a specific keyword but uses a special character the caret type TPointerToInt AInteger Once you have defined a pointer variable you can assign to it the address of another variable of the same type using the operator var P AInteger X Integer begin P QX change the value in two
63. e the Tab key to enter a non printable character When you have doubts about a programming construct writing a very sim ple test program such as this can help you learn a lot You can consider more options and Boolean expressions and increase the complexity of this small example making any test you like Marco Cantu Essential Pascal Chapter 5 Statements 63 Case Statements If your if statements become very complex at times you can replace them with case statements A case statement consists of an expression used to select a value a list of possible values or a range of values These values are constants and they must be unique and of an ordinal type Eventually there can be an else statement that is executed if none of the labels correspond to the value of the selector Here are two simple examples part ofthe CaseTest project case Number of 1 Text One 2 Text Two 3 Text rasi end case aChar of Text Plus sign T Text Minus sign wet loy ext i Multiplication or division Q 9 Text Number a az rs Text s Lowercase character A Z Text Uppercase character else qct Unknown character achar end It is considered a good practice to include the else part to signal an unde fined or unexpected condition Loops in Pascal The Pascal language has the typical repetitive statements of most program ming languages including for while and repeat statemen
64. ed in Delphi 4 and covered in Chapter 8 Dynamic arrays use the same syntax as open arrays with the difference that you can use a notation such as array of Integer to declare a variable not just to pass a parameter Type Variant Open Array Parameters Besides these typed open arrays Delphi allows you to define type variant or untyped open arrays This special kind of array has an undefined number of values which can be handy for passing parameters Technically the construct array of const allows you to pass an array with an undefined number of elements of different types to a routine at once For example here is the definition of the Format function we ll see how to use this function in Chapter 7 covering strings function Format const Format string const Args array of const string The second parameter is an open array which gets an undefined number of values In fact you can call this function in the following ways N 20 S Total writeln Format 7ota d N writeln Format znt d Float f N 12 4 writeln Format s Zd S N 2 Notice that you can pass a parameter as either a constant value the value of a variable or an expression Declaring a function of this kind is simple but how do you code it How do you know the types of the parameters The val Marco Cantu Essential Pascal Chapter 6 Procedures and Functions 77 ues of a type variant open array parameter are compa
65. em Instead of moving on to object oriented features however the next few chapters give you some details on other Pascal programming elements start ing with strings Marco Cantu Essential Pascal 88 Chapter 6 Procedures and Functions Marco Cantu Essential Pascal Chapter 7 Handling Strings 89 Chapter 7 Handling Strings String handling in Pascal looks fairly simple but behind the scenes the situa tion is quite complex Pascal has a traditional way of handling strings Windows has its own way borrowed from the C language and modern ver sions of Pascal include a powerful long string data type which is now the default string type in Delphi Types of Strings In the early days of Borland s Turbo Pascal and in 16 bit Delphi the typical string type was a sequence of characters with a length byte at the beginning indicating the current size of the string Because the length is expressed by a single byte it cannot exceed 255 characters a very low value that creates many problems for string manipulation Each string is defined with a fixed size which by default is the maximum 255 although you can declare shorter strings to save memory space Marco Cantu Essential Pascal 90 Chapter 7 Handling Strings A string type is similar to an array type In fact a string is almost an array of characters This is demonstrated by the fact that you can access a specific string character using the notation similar to the
66. emory allocations For example when you request a very large array its memory is reserved but not allocated Setting the length of a string is seldom necessary The only case in which you must allocate memory for the long string using SetLength is when you have to pass the string as a parameter to a Windows API function after the proper typecast as I ll show you shortly Looking at Strings in Memory To help you better understand the details of memory management for strings I ve written the simple StrRef example In this program I declare two global strings Str1 and str2 The program assigns a constant string to the first of the two variables and then assigns the second variable to the first Stri Hello Str2 Strl Besides working on the strings the program shows their internal status in a list box using the following StringStatus function function StringStatus const Str string string begin Result Address IntToStr Integer Str Length IntToStr Length Str References IntToStr PInteger Integer Str 8 A value Str end It is vital in the StringStatus function to pass the string parameter as a const parameter Passing this parameter by copying will cause the side effect of having one extra reference to the string while the function is being Marco Cant Essential Pascal Chapter 7 Handling Strings 93 executed By contrast passing the parameter via a referen
67. end of the line The first form is shorter and more commonly used The second form was often preferred in Europe because many European keyboards lacked the brace symbol The third form of comment has been borrowed from C and Marco Cantu Essential Pascal Chapter 2 Coding in Pascal 17 was added in Delphi 2 Comments up to the end of the line are very helpful for short comments and for commenting out a single line of code Notice that in the code listings in the book I ll try to typeset comments in italics and keywords in bold to be consistent with the default Delphi syntax highlighting and that of most other editors Having three different forms of comments can be helpful for marking nested comments If you want to comment out several lines of source code to dis able them and these lines contain some real comments you cannot use the same comment identifier code icomment creating problems code With a second comment identifier you can write the following code which is correct 5x code this comment is OK code Note that if the open brace or parenthesis star is followed by the dollar sign it becomes a compiler directive as in x Valid compiler directives are listed in the compiler documentation or help They generally affect the way the compiler generates code and are compiler specific certainly not part of the language as such and too advanced to be covered here Use of Uppercase Unlik
68. ent from the next one The only restriction in splitting programming statements on different lines is that a string literal may not span several lines Again there are no fixed rules on the use of spaces and multiple line state ments just some rules of thumb e The Delphi editor and many others have a vertical line you can place after 60 or 70 characters If you use this line and try to avoid surpassing this limit your source code will look better when you print it on paper Other wise long lines may get broken at any position when you print them e When a function or procedure has several parameters it is common prac tice to place the parameters on different lines e You can leave a line completely white blank before a comment or to divide a long piece of code in smaller portions Even this simple idea can improve the readability of the code both on screen and when you print it Use spaces to separate the parameters of a function call and maybe even a space before the initial open parenthesis Also keep operands of an expression separated I know that some programmers will disagree with these ideas but I insist Spaces are free you don t pay for them Pretty Printing The last suggestion on the use of white spaces relates to the typical Pascal language formatting style known as pretty printing This rule is simple Each time you need to write a compound statement indent it two spaces not a tab like a C programmer would
69. es defined in the Pascal language and in the Delphi system unit that work on ordinal types They are shown in the following table Dec Decrements the variable passed as parameter by one or by the value of the optional second parameter Inc Increments the variable passed as parameter by one or by the specified value Odd Returns True if the argument is an odd number Pred Returns the value before the argument in the order deter mined by the data type the predecessor succ Returns the value after the argument the successor ord Returns a number indicating the order of the argument within the set of values of the data type Low Returns the lowest value in the range of the ordinal type passed as its parameter High Returns the highest value in the range of the ordinal data type Notice that some of these routines when applied to constants are automati cally evaluated by the compiler and replaced with their value For example if 14 C programmers should notice that the two versions of the Inc procedure with one or two parameters correspond to the and operators the same holds for the Dec procedure Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 39 you call High X where X is defined as an Integer the compiler replaces the expression with the highest possible value of the Integer data type Real Types Real types represent floating point numbers in various formats Here is a l
70. es mentioned in the book is freely available The code has the same copyright as the book Feel free to use it at will but don t publish it on other documents or site Download details and a list of examples are available in the Appendix to the book Feedback Please let me know of any errors you find but also of topics not clear enough for a beginner Also let me know which other topics you d like to see me cover in future books The preferred way of sending feedback is on my public newsgroup see my web site delphi newswhat com for a web interface to my groups in the area devoted to books If you have trouble using the newsgroups email me at marco cantu gmail com Acknowledgments That I m publishing a book on the web for free is mainly due to Bruce Eckel s experience with Thinking in Java I m a friend of Bruce and think he really did a great job with that book and few others not only because of the high quality content of those books but also for his experiments with the books delivery model When I mentioned the project to people at Borland as it was called at the time I got a lot of positive feedback as well And of course I must thank the company for making first the Turbo Pascal series of compilers and now the Delphi series of visual IDEs Marco Cantu Essential Pascal 6 Introduction I ve received some precious feedback on early versions of the ebook The first readers who helped improve this material were Charles W
71. estion about 7143 1562 in the range of a floating point data type Although you can indeed know whether one real number has a higher value than another it makes no sense to ask how many real numbers exist before a given number this is the meaning of the Ord function Real types have a limited role in the user interface portion of the code the Windows side but they are fully supported by Delphi including the database side The support of IEEE standard floating point types makes the Object Pascal language completely appropriate for the wide range of pro grams that require numerical computations If you are interested in this aspect you can look at the arithmetic functions provided by the compiler s system unit see the compiler documentation or use the help system for more details Delphi and FreePascal also have a Math unit that defines advanced mathe matical routines covering trigonometric functions such as the ArcCosh function finance such as the InterestPayment function and statistics 16 Both FreePascal and GNU Pascal include the Comp data type It is correctly considered an Integer type in GNU Pascal FreePascal includes the Currency data type Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 41 such as the MeanAndStdDev procedure There are a number of these rou tines some of which sound quite strange to me such as the MomentSkew Kurtosis function I ll let you find out what this is
72. f the ASCII char acter set those with a value of 128 or more The program takes two file names for the input and output file as parame ters plus one of these extra flags U uppercase C capitalize R remove symbols At the beginning the program parses the command line parameters looking for the flags and for the input and output file names for I 1 to ParamCount do begin if Paramstr i 1 then Flag ParamStr i 2 else if inputFile then inputFile ParamStr i else outputFile ParamStr i end As parameters are compulsory before executing the actual operation required the program has this test on the input parameters if CinputFile or outputFile or not Flag in U R C then begin writeln Missing or wrong parameters readln Exit end The real code of the example is in the three conversion routines that are called depending on the parameters The routines are in a secondary unit FilterRoutines pas These calls take place inside a case statement part of the DoConvert procedure case Flag of U ConvUpper Marco Cantu Essential Pascal 132 Chapter 12 Files in the Pascal Language C ConvCapitalize R ConvSymbols end Once again you can see the entire source code among the download sample programs of the book The DoConvert procedure does most of the work related to handling the files it opens the input file as a file of bytes a file stori
73. following code Forml Canvas Pen width 21 Forml Canvas Pen Color clRed But it is certainly easier to write this code with Forml Canvas Pen do begin width 2 Color clRed end When you are writing complex code the wi th statement can be effective and spares you the declaration of some temporary variables but it has a draw Marco Cant Essential Pascal Chapter 5 Statements 69 back It can make the code less readable particularly when you are working with different enums records and objects that have similar or corresponding properties A further drawback is that using the wi th statement can allow subtle logic errors in the code that the compiler will not detect This example shows how a logic error can be correctly missed by the compiler when with is used in conjunction with derived objects with Buttonl do begin width 200 Caption New caption Color clRed end This code changes the Caption and the width of the button but it affects the Color property of the form not that of the button The reason is that the TButton components don t have the Color property and since the code is executed for a form object we are writing a method of the form this object is accessed by default If we had instead written Buttonl width 200 Buttonl Caption New Caption Buttonl Color clRed error the compiler would have issued an error In general we can say that since the with s
74. ge approach is certainly still an interesting technique I strongly urge you to use streams the TStream and derived classes to handle any complex files in Object Pascal Streams represent virtual files which can be mapped to physi Marco Cantu Essential Pascal 134 Chapter 12 Files in the Pascal Language cal files to a memory block to a socket or any other continuous series of bytes You can find more on streams in the Delphi help file and in my Mas tering Delphi book series Files management is a very large topic and even sticking to the traditional Pascal techniques for managing files one could write an entire book on this subject But I m out of space This is in fact the last chapter of the book Marco Cantu Essential Pascal PostFace 135 PostFace At least for the moment this chapter on files is the last ofthe book Feel free to send me feedback as suggested in the introduction newsgroup or email sending me your comment and requests If after this introduction on the Pascal language you want to delve into the object oriented elements of Object Pascal in Delphi you can refer to my printed book of the Mastering Delphi series like Mastering Delphi 7 or Mastering Delphi 2005 published by Sybex and my Delphi 2007 Handbook available on Lulu com Buying the printed version of this book or any of my other printed books maybe directly on Lulu or through the Amazon links on my web site is the best way to support
75. generally do to the right of the cur rent statement A compound statement inside another compound statement is indented four spaces and so on if then statement if then begin statementl statement2 end Marco Cantu Essential Pascal 20 Chapter 2 Coding in Pascal if then begin if then statement1 statement 2 end The above formatting is based on pretty printing but programmers have dif ferent interpretations of this general rule Some programmers indent the begin and end statements to the level of the inner code some of them indent begin and end and then indent the internal code once more other programmers put the begin in the line of the if condition in a C like fash ion This is mostly a matter of personal taste There are Delphi add in programs you can use to convert an existing source code to the indentation format you prefer A similar indentation format is often used for lists of variables or data types type Letters set of Char var Name string Indentation is also used for statements that continue from the previous line MessageDlg 7his 7s a message mtInformation mbOk 0 Of course any such convention is just a suggestion to make the code more readable to other programmers and it is completely ignored by the compiler I ve tried to use this rule consistently in all of the samples and code frag ments in this book Delphi source code manuals and Help examples use a
76. h you can find in the example code The conversion is based on a case statement with four branches e Ifthe letter is uppercase and it is the first letter after an ending punctuation mark as indicated by the Period Boolean variable it is left as is otherwise it is converted to lowercase This conversion is not done by a standard procedure simply because there isn t one for single characters It s done with a low level function I ve written called LowCase e Ifthe letter is lowercase it is converted to uppercase only if it was at the beginning of a new sentence e Ifthe character is an ending punctuation mark period question mark or exclamation mark Period is set to True e Ifthe character is anything else it is simply copied to the destination file and Period is set to False The following output shows an example of the transformation produced by this program tnputtext txt this is a red brown fox the fox is under a tree GOOD for the fox outputtext txt This is a red brown fox The fox is under a tree Good for the fox This program is far from adequate for professional use but it is a first step toward building a full scale case conversion program Its biggest drawbacks are that it frequently converts proper nouns to lowercase and capitalizes any letter after a period even if it s the first letter of a filename extension Summary Although direct handling of files using the traditional Pascal langua
77. have a default parameter other than ni1 records cannot be used at all e Default parameters must be passed by value or as const A refer ence var parameter cannot have a default value Using default parameters and overloading at the same time can cause quite a few problems as the two features might conflict For example if I add to the previous example the following new version of the NewMessage procedure procedure NewMessage Str string I Integer 0 overload begin writeln Str IntToStr I end then the compiler won t complain as this is a legal definition However the call NewMessage Hello is flagged by the compiler as NewMessageTest dpr 24 E2251 Ambiguous overloaded call to NewMessage Marco Cantu Essential Pascal Chapter 6 Procedures and Functions 87 Notice that this error shows up in a line of code that compiled correctly before the new overloaded definition In practice we have no way to call the NewMessage procedure with one string parameter as the compiler doesn t know whether we want to call the version with only the string parameter or the one with the string parameter and the integer parameter with a default value When it has a similar doubt the compiler stops and asks the program mer to state his or her intentions more clearly Summary Writing procedure and functions is a key element of programming and even methods their OOP counterpart share most of the features with th
78. he C language concept of function pointer The declaration of a procedural type indicates the list of parameters and in the case of a function the return type For example you can declare a new procedural type with an Integer param eter passed by reference with this code type TIntProc procedure var Num Integer This procedural type is compatible with any routine having exactly the same parameters or the same function signature to use C jargon Here is an example of a compatible routine procedure DoubleThevalue var Value Integer begin Value Value 2 end Procedural types can be used for two different purposes you can declare variables of a procedural type or pass a procedural type that is a function pointer as parameter to another routine Given the preceding type and pro cedure declarations you can write this code var IP TIntProc X Integer begin IP Doublerhevalue Modi bs IP X end This code has the same effect as the following shorter version 40 In the 16 bit version of Delphi routines must be declared using the far directive in order to be used as actual values of a procedural type Marco Cantu Essential Pascal 82 Chapter 6 Procedures and Functions var X Integer begin X t 5 DoubleThevalue OO end The first version is clearly more complex so why should we use it In some cases being able to decide which function to call and actually calling it later on ca
79. he compiler must have already seen some sort of declaration to know what the identifier refers to For this reason you usually provide a full declaration before using any rou tine However there are cases in which this is not possible If procedure A calls procedure B and procedure B calls procedure A when you start writing Marco Cantu Essential Pascal 8o Chapter 6 Procedures and Functions the code you will need to call a routine for which the compiler still hasn t seen a declaration If you want to declare the existence of a procedure or function with a certain name and given parameters without providing its actual code you can write the procedure or function followed by the forward keyword procedure Hello forward Later on the code should provide a full definition of the procedure but this can be called even before it is fully defined Here is a silly example just to give you the idea procedure DoubleHello forward procedure Hello begin if MessageDlg Do you want a double message mtConfirmation mbves mbNo 0 mrYes then DoubleHello else ShowMessage He o end procedure DoubleHello begin Hello Hello end This approach allows you to write mutual recursion Doub1eHel 10 calls Hello but Hello might call DoubleHello too Of course there must be a condition to terminate the recursion to avoid a stack overflow You can find this code with some slight changes in the DoubleH example
80. he project options however the default is generally sufficient If you ever receive a stack full error message this is probably because you have a function recursively calling itself forever not because the stack space is too limited The Heap Memory The term Heap indicates a portion of memory available to a program also called the dynamic memory area The heap is the area in which the alloca tion and deallocation of memory happens in random order This means that if you allocate three blocks of memory in sequence they can be destroyed later on in any order The heap manager or memory manager takes care of all the details for you so you simply ask for new memory with GetMem or by calling a constructor to create an object and the runtime will return you a new memory block optionally reusing memory blocks already discarded Marco Cantu Essential Pascal Chapter 8 Memory 101 Pascal uses the heap for allocating the memory of each and every object the text of the strings the content of dynamic arrays and for specific custom requests of dynamic memory Windows for example allows an application to have up to 2 GigaBytes of address space most of which can be used by the heap Dynamic Arrays Traditionally the Pascal language has always had fixed size arrays When you declare a data type using the array construct you have to specify the number of elements of the array As expert programmers probably know there were a few tec
81. hlight specific elements of Pascal operators You can see a list ofthe operators of the language below grouped by precedence Unary Operators Highest Precedence Address of variable or function returns a pointer not Boolean or bitwise not Multiplicative and Bitwise Operators E Arithmetic multiplication or set intersection Floating point division div Integer division Marco Cantu Essential Pascal 26 Chapter 2 Coding in Pascal mod as and shl shr Additive Operators or xor Modulus the remainder of integer division Allows a type checked conversion at runtime Boolean or bitwise and Bitwise left shift Bitwise right shift Arithmetic addition set union string concatenation pointer offset addition Arithmetic subtraction set difference pointer offset subtraction Boolean or bitwise or Boolean or bitwise exclusive or Relational and Comparison Operators Lowest Precedence lt gt is Test whether equal Test whether not equal Test whether less than Test whether greater than Test whether less than or equal to or a subset of a set Test whether greater than or equal to or a superset of a set Test whether the item is a member of the set Test whether object is compatible with a specified type definition which is only of use in object ori ented programming Contrary to most other programming languages the and and or operators have higher precedence than comparison ones
82. hniques you could use to implement dynamic arrays typically using pointers and manually allocating and freeing the required memory Delphi 4 introduced a very simple implementation of dynamic arrays mod eling them after the dynamic long string type I ve just covered As long strings dynamic arrays are dynamically allocated and reference counted but they do not offer a copy on write technique You can deallocate an array by setting its variable to ni 1 or its length to zero You can simply declare an array without specifying the number of elements and then allocate it with a given size using the SetLength procedure The same procedure can also be used to resize an array without losing its con tent There are also other string inspired procedures such as the Copy function that you can use on arrays Here is a small code excerpt under scoring the fact that you must both declare and allocate memory for the array before you can start using it var Arrayl array of Integer begin Arrayl 1 100 77 error SetLength Arrayl 100 Arrayl 99 100 77 OK end As you indicate only the number of elements of the array the index invari ably starts from 0 Generic arrays in Pascal allow a non zero low bound and non integer indexes two features that dynamic arrays don t support To learn about the status of a dynamic array you can use the Length High Marco Cantu Essential Pascal 102 Chapter 8 Memory and Low functions as with a
83. iary Among other changes CodeGear brought back the original name of the language which is now once again officially called Object Pascal Most notably CodeGear pro Marco Cantu Essential Pascal 14 Chapter 1 Short History of the Pascal Language duced a solid Delphi 2007 after a not so great Delphi 2005 and a some what unstable Delphi 2006 Among other Delphi dialects the two most commonly used are FPC Free Pascal Compiler and Chrome a NET based language by RemObjects I ll often mention these two dialects in the book The respective web sites are http ww freepascal org http ww remobjects com chrome Marco Cantu Essential Pascal Chapter 2 Coding in Pascal 15 Chapter 2 Coding In Pascal This chapter describes the element of a Pascal program like keywords white spaces and expressions Here you ll find the basic building blocks of Pascal As a starting point I m going to show you the code of a simple Hello World application showing some of the structural elements of a Pascal program I won t explain what these elements mean just yet as that is the purpose of the first few chapters of the book Here is the code program EssHello APPTYPE CONSOLE var strMessage string begin strMessage Hello Small world writeln strMessage watt until Enter is pressed readln end Marco Cantu Essential Pascal 16 Chapter 2 Coding in Pascal You can see the program name in the f
84. ich might be an optional error code or something like that or you can call a procedure which passes back a result within its parameters more on reference parameters later in this chapter Here are the definitions of a procedure and two versions of the same func tion using a slightly different syntax procedure Hello begin writeln C He o world end function Double Value Integer Integer begin Double Value 2 end or as an alternative function Double2 value Integer Integer begin Result Value 2 end The use of Result instead of the function name to assign the return value of a function is becoming quite popular and tends to make the code more read able Once these routines have been defined you can call them one or more times You call the procedure to make it perform its task and call a function to compute the value call the procedure Hello call the function X Double 100 Y Double X writeln IntToStr Y This is the encapsulation code concept I ve introduced before in practice When you call the Double function you don t need to know the algorithm used to implement it If you later find out a better way to double numbers you can easily change the code of the function but the calling code will remain unchanged although executing it will be faster The same principle can be applied to the Hello procedure We can modify the program output by Marco Cant
85. im ply extended from the Object Pascal language already in use in the Borland Pascal products But Borland didn t invent Pascal it only helped make it very popular and extended it a little Marco Cantu Essential Pascal 12 Chapter 1 Short History of the Pascal Language Wirth s Pascal The Pascal language was originally designed in 1971 by Niklaus Wirth pro fessor at the Polytechnic of Zurich Switzerland Pascal was designed as a simplified version for educational purposes of the Algol language which dates from 1960 When Pascal was designed many programming languages existed but few were in widespread use FORTRAN Assembler COBOL The key idea of the new language was order managed through a strong concept of data types declaration of variables and structured program controls The language was also designed to be a teaching tool Turbo Pascal Borland s world famous Pascal compiler called Turbo Pascal was intro duced in 1983 implementing Pascal User Manual and Report by Jensen and Wirth The Turbo Pascal compiler has been one of the best selling series of compilers of all time and made the language particularly popular on the PC platform thanks to its balance of simplicity and power Turbo Pascal introduced an Integrated Development Environment IDE where you could edit the code in a WordStar compatible editor run the compiler see the errors and jump back to the lines containing those errors It sounds
86. in many places For example the style of the border of a form is defined as follows type TFormBorderStyle bsNone bsSingle bsSizeable bsDialog bssizeToolwin bsToolwindow Set Types Set types indicate a group of values where the list of available values is indi cated by the ordinal type the set is based onto These ordinal types are usually limited and quite often represented by an enumeration or a sub range If we take the subrange 1 3 the possible values of the set based on it include only 1 only 2 only 3 both 1 and 2 both 1 and 3 both 2 and 3 all the three values or none of them A variable usually holds one of the possible values of the range of its type A set type variable instead can contain none one two three or more values of the range It can even include all of the values Here is an example of a set type TLetters set of TUppercase Now I can define a variable of this type and assign to it some values of the original type To indicate some values in a set you write a comma separated list enclosed within square brackets The following code shows the assign ment to a variable of several values a single value and an empty value var Lettersl Letters2 Letters3 TLetters begin Lettersl A ei c Letters2 K Letters3 21 You can change the default representation of enumerated types asking for a larger one by using the Z compiler directive 22 When the v
87. in the StringAndPChar example Marco Cantu Essential Pascal 96 Chapter 7 Handling Strings Formatting Strings Using the plus operator and some of the conversion functions such as IntToStr you can indeed build complex strings out of existing values However there is a different approach to formatting numbers currency val ues and other strings into a final string You can use the powerful Format function or one of its companion functions The Format function requires as parameters a string with the basic text and some placeholders usually marked by the symbol and an array of values one for each placeholder For example to format two numbers into a string you can write Format F7rst d Second d nl n2 where n1 and n2 are two Integer values The first placeholder is replaced by the first value the second matches the second and so on Ifthe output type of the placeholder indicated by the letter after the symbol doesn t match the type of the corresponding parameter a runtime error occurs Having no compile time type checking is actually the biggest drawback of using the Format function Similarly not passing enough parameters causes a run time error The Format function uses an open array parameter a parameter that can have an arbitrary number of values as covered in Chapter 6 Besides using d you can use one of many other placeholders defined by this function and briefly listed the following tabl
88. ince they are hidden inside forms bitmaps and other Delphi objects They become useful when you want to call a Windows API function that is not directly supported by Delphi Marco Cantu Essential Pascal Chapter 9 Windows Programming 107 External Declarations Another important element for Windows programming is represented by external declarations Originally used to link the Pascal code to external functions that were written in assembly language external declarations are used in Windows programming to call a function from a DLL a dynamic link library In Delphi there are a number of such declarations in the Win dows unit forward declaration function GetUserName lpBuffer PChar var nSize DWORD BOOL stdcall external declaration instead of actual code function GetUserName external advapi32 name GetUserNnameA You seldom need to write declarations like the one just illustrated since they are already listed in the Windows unit and many other Delphi system units The only reason you might need to write this external declaration code is to call functions from a custom DLL or to call undocumented Windows func tions This declaration means that the code of the function GetUserName is stored in the advapi32 dynamic library advapi32 is a constant associated with the full name of the DLL advapi32 dIl with the name GetuserNameaA as this function has both an ASCII and a WideString version Inside an exte
89. ions As you type the open parenthesis after the routine name all the available alternatives are listed As you enter the parameters Delphi uses their type to determine which of the alternatives are still available 42 For more information on units see Chapter 11 Marco Cant Essential Pascal Chapter 6 Procedures and Functions 85 For example you can add to a program in this case the OverDef example the following code procedure MessageBox str string overload begin Windows MessageBox 0 PChar str 77t e MB OK end This code doesn t overload the original MessageBox routine of the Windows API at all In fact if you try to call the original version with MessageBox 0 Message Title MB OK you ll get a nice error message indicating that some of the parameters are missing The only way to call the official version instead of the local one is to refer explicitly to the unit something that defeats the idea of overloading windows MessageBox 0 Message Title MB OK Default Parameters Another feature related to overloading is that you can give a default value to the parameter or parameters of a function so that you can call the function with or without the parameter If the parameter is missing in the call it will take the default value Let me show an example We can define the following encapsulation of the write call providing two default parameters procedure NewMessage Msg st
90. irst line a compiler directive a vari able declaration and three lines of code plus a comment within the main begin end block Again we ll learn about all of these elements soon this serves only to give you an idea of what a small but complete Pascal program looks like Syntax and Style Before we move on to the subject of writing Pascal language statements it is important to highlight a couple of elements of Pascal coding style The ques tion I m addressing here is this Besides the syntax rules how should you write code There isn t a single answer to this question since personal taste can dictate different styles However there are some principles you need to know regarding comments uppercase spaces and the so called pretty printing pretty for us human beings not the computer In general the goal of any coding style is clarity The style and formatting decisions you make are a form of shorthand indicating the purpose of a given piece of code An essential tool for clarity is consistency whatever style you choose be sure to follow it throughout a project and across projects Comments In traditional Pascal comments were enclosed in either braces or parenthe ses followed by a star Modern versions also accept the C style comments double slash which can span to the end of the line and has no symbol to sig nal the end the comment this 7s a comment this is another comment this is a comment up to the
91. ist of floating point data types Single The smallest storage size is given by Single numbers which are implemented with a 4 byte value Double These are floating point numbers implemented with 8 bytes Extended These are numbers implemented with 10 bytes These are all floating point data types with different precision which corre spond to the IEEE standard floating point representations and are directly supported by the CPU for maximum speed In Delphi 2 and Delphi 3 the Real type had the same definition as in the 16 bit version it was a 48 bit type But its usage was deprecated by Borland who suggested that you use the Single Double and Extended types instead The reason for their suggestion is that the old 6 byte format is nei ther supported by the Intel CPU nor listed among the official IEEE real types To completely overcome the problem Delphi 4 modified the definition of the Real type to represent a standard 8 byte 64 bit floating point num ber In addition to the advantage of using a standard definition this change allows components to publish properties based on the Real type something Delphi 3 did not allow Among the disadvantages there might be compatibil ity problems If necessary you can overcome the possibility of incompatibility by sticking to the Delphi 2 and 3 definition of the type do this by using the following compiler option REALCOMPATIBILITY ON There are also two strange non ordinal data ty
92. k better suited for different platforms and compilers Changing the examples from visual ones to console based ones brings with it the advantage that the reader can focus even more on the lan guage ignoring event handlers methods component and other more advance topics Also the programs can run on non Windows platforms Book Copyright and Availability The text and the source code of this book is copyright of Marco Cant Of course you can use the programs and adapt them to your own needs only you are not allowed to use them without permission in books training mate rial and other copyrighted formats unless of course you use a reasonably limited amount of the text or source code and explicitly mention the source Distributing this book in electronic format is not allowed The HTML version is available and will remain available free of charge at http www marcocantu com epascal In the very beginning the book was HTML only Following editions were available as PDF which was a free download although I did ask for a volun teer based license fee or donation Now I m publishing the book in three formats with exactly the same content e Free HTML version with advertising e PDF version you can buy on Lulu com if you ve obtained it else where this is probably an illegal copy Marco Cant Essential Pascal Introduction 5 e Printed version you can buy on Lulu com Source Code The source code of all the exampl
93. l Pascal to use a specific type you can simply add the type name in the declaration as in const Thousand Integer 1000 When you declare a constant the compiler can choose whether to assign a memory location to the constant and save its value there or to duplicate the actual value each time the constant is used This second approach makes sense particularly for simple constants Marco Cantu Essential Pascal 32 Chapter 3 Types Variables and Constants Like Turbo Pascal the 16 bit version of Delphi allowed you to change the value of a typed constant at run time as if it was a variable The 32 bit ver sions still permits this behavior for backward compatibility when you enable the 3 compiler directive or use the corresponding Assignable typed con stants check box of the Compiler page of the Project Options dialog box This setting was on by default until Delphi 6 but in any case you are strongly advised not to use this trick as a general programming technique Assigning a new value to a constant disables all the compiler optimizations on con stants In such a case simply declare a variable instead Resource String Constants When you define a string constant instead of writing a standard constant declaration you can use a specific directive resourcestring that indicates to the compiler and linker to treat the string like a Windows resource const sAuthorName Marco resourcestring strAuthorName Marco
94. ler at automated begin case cdecl class const constructor contains default destructor dispid dispinterface div do downto dynamic else end except export exports external far file finalization finally for forward function goto if implementation implements in index inherited initialization inline interface is label library message mod name near nil nodefault not object of on or out over load override package packed pascal private procedure program property protected public published raise read readonly record register reintroduce repeat requires resident resourcestring safecall set shl shr stdcall stored string then threadvar Marco Cant Essential Pascal Chapter 2 Coding in Pascal 23 to try type unit until uses var virtual while with write writeonly xor FreePascal has a few extra reserved words dispose exit false new true Literal Values A literal value is a value you type directly in the program source code If you need a number with the value of two you simply enter 2 This will be the literal value for an integer number If you want the same value but for a floating point literal value you generally add an empty deci mal after it 2 0 Literal values are not limited to numbers You can also have characters and strings Both use single quotes literal characters K literal string Marco You can also indicate characters by their ASCII number prefixing the num ber with the sy
95. lt since Delphi 3 is indicated by the register keyword Marco Cant Essential Pascal Chapter 6 Procedures and Functions 79 The problem is that this is the default convention and functions using it are not compatible with Windows the functions of the Win32 API must be declared using the stdca11 calling convention a mixture of the original pascal calling convention of the Win16 API and the cdec1 calling conven tion of the C language There is generally no reason not to use the new fast calling convention unless you are making external Windows calls or defining Windows callback functions We ll see an example using the stdcal1 convention before the end of this chapter You can find a summary of Delphi calling conventions in the Calling conventions topic under Delphi help What Is a Method If you have already worked with Delphi or read the manuals you have prob ably heard about the term method A method is a special kind of function or procedure that is related to a data type a class In Delphi every time we handle an event we need to define a method generally a procedure In gen eral however the term method is used to indicate both functions and procedures related to a class Here is an empty method automatically added by Delphi to the source code of a form procedure TForml ButtonlClick sender TObject begin here goes your code end Forward Declarations When you need to use an identifier of any kind t
96. lty for using it is that it uses much much more memory 18 Delphi 1 0 used a different zero point for TDateTime values Marco Cantu Essential Pascal 42 Chapter 3 Types Variables and Constants DateTimeToStr DateTimeToString DateToStr TimeToStr FormatDateTime StrToDateTime StrToDate StrToTime DayOfweek DecodeDate DecodeTime EncodeDate EncodeTime Converts a date and time value into a string using default formatting to have more control on the con version use the FormatDateTime function instead Copies the date and time values into a string buffer with default formatting Converts the date portion of a date time value into a string Converts the time portion of a date time value into a string Formats a date and time using the specified format you can specify which values you want to see and which format to use by providing a complex format string Converts a string with date and time information to a date time value raising an exception in case of an error in the format of the string Its companion func tion StrToDateTimeDef returns the default value in case of an error rather than raising an exception Converts a string representing a date into a date time value Converts a string representing a time into a date time value Returns the number corresponding to the day of the week of the date time value passed as parameter Retrieves the year month and day values from a
97. lue truncating its value Returns the Integer part of the floating point value argument Converts a number into a string Converts a number into a string with its hexadecimal representation Converts a string into a number raising an exception if the string does not represent a valid integer Converts a string into a number using a default value if the string is not correct Converts a string into a number traditional Turbo Pascal routine available for compatibility Converts a number into a string using formatting parameters traditional Turbo Pascal routine avail able for compatibility Converts a null terminated string into a Pascal style string This conversion is automatically done for AnsiStrings in 32 bit Delphi See the section on strings later in this chapter Copies a Pascal style string into a null terminated string This conversion is done with a simple PChar Marco Cant Essential Pascal 46 Chapter 3 Types Variables and Constants StrPLCopy FloatToDecimal FloatToStr FloatToStrF FloatToText FloatToTextFmt StrToFloat TextToFloat cast in 32 bit Delphi See the section on strings later in this chapter Copies a portion of a Pascal style string into a null terminated string Converts a floating point value to record including its decimal representation exponent digits sign Converts a floating point value to its string represen tation using default formatting
98. mbol as I ll show in more details in the section about the Char data type in the next chapter In case you need to have a quote within a string you ll have to double it So if I want to have my first and last name spelled with a final quote rather than an accent I can write Marco Cantu vier 7 Source http www freepascal org docs html ref refsu3 html Marco Cantu Essential Pascal 24 Chapter 2 Coding in Pascal The two quotes stand for a quote within the string while the third consecu tive quote marks the end of the string Also note that a string literal must be written on a single line Expressions and Operators There isn t a general rule for building expressions since they mainly depend on the operators being used and Pascal has a number of operators There are logical arithmetic Boolean relational and set operators plus some oth ers sample expressions 20 30 n a b c 10 Expressions are common to most programming languages An expression is any valid combination of constants variables literal values operators and function results Expressions can be used to determine the value to assign to a variable to compute the parameter of a function or procedure or to test for a condition Every time you are performing an operation on the value of an identifier rather than using an identifier by itself you are using an expression Showing the Result of an Expression a First Program
99. my writing and push me to write more on Pascal Delphi and also other topics in the future Helping me with updates corrections and suggestions is another very good way to contribute For more information on the latest edition of Mastering Delphi the compan ion Essential Delphi and more advanced books of mine and of other authors as well you can refer to my web site http ww marcocantu com Happy Pascal coding Marco Cantu Essential Pascal 136 Appendix Examples Appendix Examples This is a list ofthe examples which are part of the Essential Pascal book and available for download in a single zip file EPasCodev3 zip about 30 KB on http www marcocantu com epascal Here is the list of examples by chapter e Chapter 2 EssHello EPExpressions Chapter 3 EPConstants EPRange TimeNow Variables Chapter 4 Pointers Chapter 5 CaseTest ForTest IfTest LoopsTest Chapter 6 OpenArr OverDef Chapter 7 StrRef FmtTest StringAndPChar Chapter 8 NewMessageTest Chapter 9 EnumTitles StrParam Chapter 10 VSpeed Chapter 12 Filter IntegersToFile StringsToFile 55 For more Pascal language demos and source code I suggest you look at the SWAG archive currently located at http www bsdg org SWAG index html Marco Cant Essential Pascal Acknowledgments esses 5 Algorithms Data Structures Programs 59 ANSIChar piani 35 ANSIStFing eene ierit ie e etc 90 Append eonim
100. n be used in several places in a program such as at the beginning of a function or procedure to declare variables local to a routine or inside a unit to declare global variables After the var keyword comes a list of variable names followed by a colon and the name of the data type You can write more than one variable name on a single line as A and B in the last statement of the previous code snippet Once you have defined a variable of a given type you can only perform the operations supported by its data type on it For example you can use the Boolean value in a test and the integer value in a numerical expression You cannot mix Booleans and integers as you can with the C language Using simple assignments we can write the following code which is part of the Variables example Value 10 IsCorrect True Given the previous variable declarations these two assignments are correct The next statement instead is not correct as the two variables have differ ent data types Value IsCorrect error If you try to compile this code the compiler issues an error with a descrip tion like this DCC Error Incompatible types Integer and Boolean Usually errors like these are programming errors because it does not make sense to assign a True or False value to a variable of the Integer data type You should not blame the compiler for these errors It only warns you that there is something wrong in your code
101. n be useful It is possible to build a complex example showing this approach However I prefer to let you explore a fairly simple one named ProcType This example is more complex than those we have seen so far to make the situation a little more realistic This example is based on two procedures One procedure is used to double the value of the parameter This procedure is similar to the version I ve already shown in this section A second procedure is used to triple the value of the parameter and therefore is named TripleThevalue procedure TripleThevalue var Value Integer begin Value Value 3 writeln va7ue tripled IntToStr value end Both procedures display what is going on to let us know that they have been called This is a simple debugging feature you can use to test whether or when a certain portion of code is executed instead of using a breakpoint Instead of calling this functions directly to demonstrate the use of procedu ral types I ve instead used a longer but more interesting approach Each time a user enters 2 or 3 on the keyboard one of the procedures is stored in a variable var ch Char IP TIntProc X Integer begin initial defaults X 10 IP DoubleThevalue while True do begin read and cal read ch Marco Cant Essential Pascal Chapter 6 Procedures and Functions 83 case ch of 2 IP DoubleThevalue 3 IP TripleThevalue x Break else I
102. n that global memory area until the program termi nates even if they are used for a much more limited amount of time This is why global memory usage is generally very limited in Pascal and Delphi applications favoring the two dynamic memory areas applications can use the heap and the stack Marco Cantu Essential Pascal 100 Chapter 8 Memory The Stack Memory The term Stack indicates a portion of memory available to a program which is dynamic but is allocated and deallocated in a strict sequence Stack alloca tion is LIFO Last In First Out This means that the last memory object you ve allocated will be the first to be deleted Stack memory is typically used by routines procedure function and method calls for parameters and local variables When you call a routine its parameters and return type are placed on the stack unless you optimize the call as Delphi does by default Also the vari ables you declare within a routine using a var block before the begin statement are stored on the stack so that when the routine terminates they ll be automatically removed before getting back to the calling routine in LIFO order Pascal uses the stack for routine parameters and return values unless you set the default register calling convention for local routine variables for Windows API function calls and so on Applications can reserve a large amount of memory for the stack In Delphi you set this in the linker page of t
103. ndows you usually never see and there are also many with no caption that the program omits Marco Cantu Essential Pascal 110 Chapter 9 Windows Programming A Minimal Windows Program To complete the coverage of Windows programming and the Pascal lan guage I want to show you a very simple but complete application built without using the VCL The program simply takes the command line param eter stored by the system in the cmdLine global variable and then extracts information from it with the ParamCount and ParamStr Pascal functions The first of these functions returns the number of parameters the second returns the parameter in a given position Although users seldom specify command line parameters in a graphical user interface environment the Windows command line parameters are impor tant to the system For example once you have defined an association between a file extension and an application you can simply run a program by selecting an associated file In practice when you double click on a file Windows starts the associated program and passes the selected file as a com mand line parameter Here is the complete source code of the project program Strparam uses Windows begin show the full string MessageBox 0 cmdLine StrParam Command Line MB OK show the first parameter if ParamCount gt 0 then MessageBox 0 PChar ParamStr 1 Ist StrParam Parameter MB OK else MessageBox 0 PChar wo
104. ng data as plain bytes the first time so that it can use the FileSize procedure which is not available for text files Then this file is closed and reopened as a text file The routine manages the input and output files and then calls one of the three processing routines Now let s take a look at one of the conversion rou tines in detail The simplest of the three conversion routines is ConvUpper which converts every character in the text file to uppercase Here is its code procedure Convupper var Ch Char Position LongInt begin Position 0 while not Eof FileIn do begin Read FileIn Ch Ch UpCase ch write Fileout Ch Inc Position end end This method reads each character from the source file until the program reaches the end of the file Eof Each single character is converted and copied to the output file As an alternative it is possible to read and convert one line at a time that is a string at a time using string handling routines This will make the program significantly faster The approach I ve used here is reasonable only for an introductory example The conversion procedure for removing symbols is very simple while not Eof FileIn do begin Read FileIn Ch if Ch lt Chr 127 then Write Fileout Choose Marco Cantu Essential Pascal Chapter 12 Files in the Pascal Language 133 The procedure used to capitalize the text in contrast is really a complex piece of code whic
105. nt value is converted to a floating point string but also uses thousands sep arators The corresponding floating point value is converted to a string representing a currency amount The con version is based on regional settings see the Delphi Help file under Currency and date time formatting variables The best way to see examples of these conversions is to experiment with for mat strings yourself To make this easier I ve written the FmtTest program which allows a user to provide formatting strings for integer and floating point numbers This program has much more interaction than most pro grams in this book It first asks whether to perform either Integer of Floating number formatting calling one of two specific routines depending on the input This is the main portion of the program begin Done False while not Done do begin writeln work with I nteger or F loating point numbers I or F or x to exit readln chInput case Upcase chInput of I TestFormatInteger F TestFormatFloat X Done else True Marco Cantu Essential Pascal 98 Chapter 7 Handling Strings writeln Wrong selection end end writeln Bye readln end Each of the routines asks for a numeric input and a format string providing some suggestions procedu re TestFormatInteger var n Integer strFmt string begin writeln Enter value readln n writeln Enter a format
106. ntroduced by the Pascal language was the ability to define new data types in a program Pro grammers can define their own data types by means of type definitions such as subrange types array types record types enumerated types pointer types and set types The most important user defined data type is the class which is part of the object oriented extensions of Object Pascal not covered in this book If you think that type constructors are common in many programming lan guages you are right but Pascal was the first language to introduce the idea in a formal and very precise way Marco Cantu Essential Pascal 48 Chapter 4 User Defined Data Types Named and Unnamed Types User defined data types can be given a name for later use or applied to a variable directly The convention in Delphi is to use a letter T prefix to denote any data type including classes but not limited to them I strongly suggest you to stick to this rule even if might not feel natural at first When you give a name to a type you must provide a specific section in the code such as the following type subrange definition TUppercase A Z array definition TDayTemperatures array 1 24 of Integer record definition TMyDate record Month Byte Day Byte Year Integer end enumerated type definition TColors Red Yellow Green Cyan Blue Violet set definition TLetters set of Char Similar type definitions c
107. ny other array For dynamic arrays however Low always returns 0 and High always returns the length minus one This implies that for an empty array High returns 1 which when you think about it is a strange value as it is lower than that returned by Low After this short introduction I can show you a simple example called DynArr It is indeed simple because there is nothing very complex about dynamic arrays I ll also use it to show a few possible errors programmers might make The program declares two global arrays and initializes the first as it starts var Arrayl Array2 array of Integer begin allocate SetLength Arrayl 100 This sets all the values to zero The initialization code makes it possible to start reading and writing values of the array right away without any fear of memory errors Assuming of course that you don t try to access items beyond the upper bound of the array For an even better initialization the program has further code that writes into each cell of the array var I Integer begin for I Low Arrayl to High Arrayl do Arrayl I I By calling SetLength again you can modify the size of the array without losing its contents You can test this reading a value grow keeping existing values SetLength Arrayl 200 extract writeln IntToStr Arrayl 99 The only slightly complex code is in the final part of the program which copies one array to the other one with the
108. ong lists of such statements programs are usually split into routines procedures or functions This is the topic of the next chapter which also introduces some advanced elements Marco Cantu Essential Pascal Chapter 6 Procedures and Functions 71 Chapter 6 Procedures And Functions Another important idea emphasized by Pascal is the concept of the routine basically a series of statements with a unique name which can be activated many times by using their name This way you avoid having to write the same statements over and over and will have a single version of the code used in many places through the program From this point of view you can think of routines as a basic code encapsulation mechanism I ll get back to this topic with an example after I introduce the Pascal routines syntax Pascal Procedures and Functions In Pascal a routine can assume two forms a procedure and a function In theory a procedure is an operation you ask the computer to perform a func tion is a computation returning a value This difference is emphasized by the Marco Cantu Essential Pascal 72 Chapter 6 Procedures and Functions fact that a function has a result a return value while a procedure doesn t Both types of routines can have multiple parameters of specified data types In practice however the difference between functions and procedures is very limited you can call a function to perform some work and then skip the result wh
109. ood and Wyatt Wong Mark Greenhaw and Frederic Gauthier Boutin helped with some editing of the text Rafael Barranco Droege offered a lot of technical correc tions and language editing Thanks While working on this new edition I had editorial help from Peter W A Wood and technical reviews by Patricio Moschcovich and Keld R Hansen The book cover was designed by Fabrizio Schiavi It represents a Pascal tri angle and matches the cover of my Delphi 2007 Handbook About the Author I live in Piacenza Italy After teaching the C language and writing C and Object Windows Library books and articles in 1995 I delved into Delphi programming I m the author of the Mastering Delphi book series published by Sybex the advanced Delphi Developers Handbook which is hard to find these days but might soon get republished by myself and the recently pub lished Delphi 2007 Handbook I have written articles for many magazines including The Delphi Magazine have spoken at Delphi and Borland confer ences around the world and given Delphi classes at basic and advanced levels Lately I ve been getting more and more involved in Web 2 0 development techniques and XML related technologies although mostly from the Delphi perspective You can find more details about me and my work on my web site http ww marcocantu com and on my blog http blog marcocantu com 2 More information on http en wikipedia org wiki Pascal s_triangle Marco
110. pe hence the name subrange For example you can define a subrange of the Integer type from 1 to 10 or from 100 to 1000 or you can define a sub range of the Char type with uppercase characters only as in type TTen 1 10 ToverHundred 100 1000 Tuppercase LAT Lei In the definition of a subrange you don t need to specify the name of the base type You just need to supply two constants of that type The original type must be an ordinal type and the resulting type will be another ordinal type When you have defined a variable as a subrange you can then assign it any value within that range This code is valid var UppLetter TUppercCase begin UppLetter F But this one is not var UppLetter TUppercCase begin i UppLetter e compile time error Marco Cantu Essential Pascal 50 Chapter 4 User Defined Data Types Writing the code above results in a compile time error Constant expres sion violates subrange bounds If you write the following code instead var UppLetter TUppercase Letter Char begin Letter e UppLetter Letter Delphi will compile it At run time if you have enabled the Range Checking compiler option in the Compiler page of the Project Options dialog box youll get a Range check error message I suggest that you turn on this compiler option while you are developing a program so it ll be more robust and easier to debug as in case of errors you ll get an ex
111. pes 15 Anewtype called Real48 was introduced for backward compatibility with the Real type of older Borland Pascal compilers Marco Cantu Essential Pascal 40 Chapter 3 Types Variables and Constants Comp describes very big integers using 8 bytes which can hold numbers with 18 decimal digits currency not available in 16 bit Delphi indicates a fixed point deci mal value with four decimal digits and the same 64 bit representation as the Comp type As the name implies the Currency data type has been added to handle very precise monetary values with four decimal places We cannot build a program similar to the EPRange example with real data types because we cannot use the High and Low functions or the Ord func tion on real type variables Real types represent in theory an infinite set of numbers ordinal types represent a fixed set of values Let me explain this better when you have the integer 23 you can determine which is the following value Integers are finite they have a determined range and they have an order Floating point numbers are infinite even within a small range and have no order in fact how many values are there between 23 and 24 And which number follows 23 46 Is it 23 47 23 461 or 23 4601 That s really impossible to know For this reason whilst it makes sense to ask for the ordinal position of the character w in the range of the Char data type it makes no sense at all to ask the same qu
112. plicit message and not an undetermined behavior You can eventually disable this option for the final build of the program so that it will run a little faster However the increase in speed is so little that I suggest you leave all these run time checks turned on even when a shipping pro gram The same holds true for other run time checking options such as overflow and stack checking Enumerated Types Enumerated types usually referred to as enums constitute another user defined ordinal type Instead of indicating a range of an existing type in an enumeration you list all of the possible values for the type In other words an enumeration is a list of values Here are some examples type TColors Red Yellow Green Cyan Blue Violet Tsuit Club Diamond Heart Spade Each value in the list has an associated ordinality starting with zero When you apply the ord function to a value of an enumerated type you get this zero based value For example Ord Diamond returns 1 Enumerated types can have different internal representations By default Delphi uses an 8 bit representation unless there are more than 256 different values in which case it uses the 16 bit representation There is also a 32 bit Marco Cantu Essential Pascal Chapter 4 User Defined Data Types 51 representation which might be useful for compatibility with C or C libraries The Delphi VCL Visual Component Library uses enumerated types
113. pointer has no value you can assign the ni1 value to it Then you can test whether a pointer is ni 1 to see if it currently refers to a value This is often used because dereferencing that is accessing the value in the address stored in the pointer an invalid pointer causes an access violation also known as a general protection fault var P AInteger begin P_ nil writeln PA You can see an example of the effect of this code by running the Pointers example after uncommenting the last few lines of code The error you ll see should be similar to Exception EAccessviolation in module Pointers exe at 000083AC Access violation at address 004083AC in module Pointers exe Read of address 00000000 One of the ways to make pointer data access safer is to add a pointer is not null safe check like the following if P lt gt nil then writeln PA An alternative way generally preferable for readability reasons is to use the Assigned pseudo function 9 28 This code should indeed use a try finally block a topic I decided not to introduce in this book because despite its relevance it was not part of the traditional Pascal language or its Turbo Pascal extensions 29 Assigned is not a real function because it is resolved by the compiler producing the proper code Marco Cant Essential Pascal 58 Chapter 4 User Defined Data Types if Assigned P then writeln PA Delphi also defines a Pointer data t
114. r MM 34 EA aN EE N a AA EEA AE ITE TEE A E E A E EEN 35 GALA E 35 Displaying Ordinal Ranges iii 36 Ordinal Types Routines eese esee eene enne tenentes tne ete rene seen arenarie eaae enean 38 Real Types ute eie eet teda eti e 39 Date oMNInTIR Ces 41 Specific Windows Types iii 44 Typecasting and Type CONVvErsions iii 44 SUMMAT sesbbune tess 46 Chapter 4 User Defined Data Types eee oe ero nee ee eee anne eere obe ore pe esee ee aa aa ES EE TE Pee Penes epos 47 Named and Unnamed Types esere senar at E KREE tn enne etnies teet tentent tenere etas 48 Secun cbe c EE 49 Enumerated N aet AA iii 50 DEL bn 51 AUC NAM BuU EC 52 Rec rd 449 T 54 INS rac ani ee ae 56 iD Mu C rene cia A TORI AR LI LN 58 COTICIUSION EE 58 Chapter 5 Statenients 2 viec ENNEN d EENS EENS EEN ENEE ENEE ENNEN E SO Simple and Compound Statements iii 59 Assignment StatementS ssrsssrscrereene sine ttn eet enne antenne anta anii aR Saoer ea Eas 60 Conditional Statements rie iii 61 If Statemerts 3 iet ener e Siate riviera 61 Case Sta terments nee ste rer E ERR E NL ee e tr 63 LOOPS in Pascali T M ern 63 Marco Cant Essential Pascal Table Contents 9 Th For EE 64 While and Repeat Statements iii 65 Examples of LOOPS 4er narici 66 The With State men oiar AOL io 67 SUMMA
115. r is used both to pass a value to the procedure and to return a new value to the calling code When you write var X Integer 36 Delphi editor helps you by suggesting the parameters list of a function or procedure with a fly by hint as soon as you type its name and the open parenthesis This feature is called Code Parameters and is part of the Code Insight technology Marco Cant Essential Pascal 74 Chapter 6 Procedures and Functions begin X 10 DoubleThevalue X the value of the X variable becomes 20 because the function uses a reference to the original memory location of X affecting its initial value Passing parameters by reference makes sense for ordinal types for old fash ioned strings and for large records Delphi strings have a slightly different behavior they behave as references but if you change one of the string vari ables referring to the same string in memory this is copied before updating it Along string passed as a value parameter behaves as a reference only in terms of memory usage and speed of the operation But if you modify the value of the string the original value is not affected On the contrary if you pass the long string by reference you can alter the original value Delphi 3 introduced a new kind of parameter out An out parameter has no initial value and is used only to return a value These parameters should be used only for COM procedures and functions in general it is better to s
116. r to focus on long strings Using Long Strings If you simply use the string data type you get either short strings or ANSI strings depending on the value ofthe H compiler directive H the default stands for long strings the ANSIString type which is what is used by the components of the Delphi library Pascal long strings are based on a reference counting mechanism which keeps track of how many string variables are referring to the same string in memory This reference counting is used also to free the memory when a string isn t used anymore that is when the reference count reaches zero Ifyou want to increase the size of a string in memory but there is something else in the adjacent memory then the string cannot grow in the same mem ory location and a full copy of the string must therefore be made in another location When this situation occurs run time support reallocates the string for you in a completely transparent way You simply set the maximum size of Marco Cantu Essential Pascal 92 Chapter 7 Handling Strings the string with the SetLength procedure effectively allocating the required amount of memory SetLength Stringl 200 The SetLength procedure performs a memory request not an actual mem ory allocation It reserves the required memory space for future use without actually using the memory This technique is based on a feature of the Win dows operating systems and is used by Delphi for all dynamic m
117. re in for a surprise the output string will be Marco This is my name followed by 15 spaces and followed by the The problem is that when Windows writes to the string within the GetUserName API call it doesn t set the length of the long Pascal string properly Pascal can still use this string for output and can figure out when it ends by looking for the C language null terminator added by Windows but it will append fur ther text at the very and and in other examples skip any text you add after the null terminator How can we fix this problem The solution is to tell the system to convert the string returned by the GetUserName API call back to a Pascal string How ever if you write the following code Strl String Strl the system will ignore it because converting a data type back into itself is a useless operation To obtain the proper long Pascal string you need to recast the string to a PChar and let Pascal convert it back again properly to a string Strl String PChar Str1 Actually you can skip the string conversion because PChar to string con versions are automatic in Pascal so you can write GetUserName PChar Strl nSize Strl PChar Strl writeln Strl and obtain the expected output Marco An alternative is to reset the length of the Pascal string using the length of the PChar string by writing SetLength Str1 StrLen PChar str1 You can find most of this code snippets
118. reover it corresponds to the native representation of numbers in the CPU Another new type introduced in Delphi 4 is the Int64 type which represents integer numbers with up to 18 digits This new type is fully supported by Marco Cant Essential Pascal Chapter 3 Types Variables and Constants 35 some of the ordinal type routines such as High and Low numeric routines such as Inc and Dec and string conversion routines such as IntToStr Boolean Boolean values other than the Boolean type are seldom used Some Boolean values with specific representations are required by Windows API functions and COM libraries The types are ByteBool wordBool and LongBool In Delphi 3 for compatibility with Visual Basic and OLE automation the data types ByteBool wordBool and LongBool were modified to represent the value True with 1 while the value False is still o The Boolean data type remains unchanged True is 1 False is 0 although the actual numeric val ues should be irrelevant and should not be abused like in C Characters Finally there are two different representation for characters ANSIChar and widechar The first type represents 8 bit characters corresponding to the ANSI character set traditionally used by Windows the second represents 16 bit characters corresponding to the new Unicode characters supported alongside with the traditional ones by recent versions of Windows Most of the time you ll simply use th
119. required as in the following begin A B A 2 C end Both versions are correct The first version has a useless but harmless semicolon This semicolon is in fact a null statement or an empty state ment that is a statement with no code Although these final semicolons serve no purpose I tend to use them and suggest you do the same Sometimes after you ve written a couple of lines you might want to add one more statement If the last semicolon is missing you have to remember to add it so it is usually better to add it in the first place Assignment Statements Assignments in Pascal use the colon equal operator 2 an odd notation for programmers who are used to other languages The operator which is used for assignments in many other languages in Pascal is used to test for equality By using different symbols for an assignment and an equality test the Pascal compiler like the C compiler can translate source code faster because it 31 Notice that at times null statements can be specifically used inside loops or in other particular cases while condition with side effect do null or empty statement 32 Except people like the book editor whose first introduction was Algol and first professional language was PL 1 Marco Cant Essential Pascal Chapter 5 Statements 61 doesn t need to examine the context in which the operator is used to deter mine its meaning The use of different operato
120. reted code because as with interpreted code many operations can not be resolved until run time In particular this affects the speed of the code Now that I ve warned you against the use of the Variant type it is time to look at what it can do Basically once you ve declared a variant variable such as the following var V Variant you can assign to it values of several different types V 10 V i Hello world V i 45 55 Once you have the variant value you can copy it to any compatible or incom patible data type If you assign a value to an incompatible data type Delphi performs a conversion if it can Otherwise it issues a run time error In fact a variant stores type information along with the data allowing a number of run time operations these operations can be handy but are both slow and unsafe Consider the following example called VariTest which is an extension of the code above var V Variant s string begin V 10 S Le Vi writeln s V Hello world S i Vj writeln s Vv i 45 55 S I V3 writeln s Funny isn t it Besides assigning a variant holding a string to the s variable you can assign to it a variant holding an integer or a floating point number Even worse you can use the variants to compute values as you can see in the following code Marco Cantu Essential Pascal Chapter 10 Variants 115 var V Variant N Integer s string begin V 208 N
121. ring Caption string Message Separator string sLineBreak begin write Caption write write Msg write Separator end With this definition we can call the procedure in each of the following ways NewMessage Something wrong here NewMessage Something wrong here Attention f NewMessage Wello Message Marco Cantu Essential Pascal 86 Chapter 6 Procedures and Functions This is the output Message Something wrong here Attention Something wrong here Message Hello Notice that Delphi doesn t generate any special code to support default parameters nor does it create multiple copies of the routines The missing parameters are simply added by the compiler to the calling code There is one important restriction affecting the use of default parameters You cannot skip parameters For example you can t pass the third parameter to the function after omitting the second one This is the main rule for default parameters In a call you can only omit parameters starting from the last one In other words ifyou omit a parame ter you must also omit the following ones There are a few other rules for default parameters as well e Parameters with default values must be at the end of the parameters list e Default values must be constants Obviously this limits the types you can use with default parameters For example a dynamic array or an interface type cannot
122. rmed into strings depending on the system s international settings The runtime reads these values from the system and copies them to a number of global variables declared in the SysUtils unit Some of them are DateSeparator Char ShortDateFormat string LongDateFormat string TimeSeparator char TimeAMString string TimePMString string ShortTimeFormat string LongTimeFormat string ShortMonthNames array 1 12 of string LongMonthNames array 1 12 of string ShortDayNames array 1 7 of string LongDayNames array 1 7 of string 19 Units and uses statements are covered in detail in Chapter 11 Marco Cantu Essential Pascal 44 Chapter 3 Types Variables and Constants More global variables relate to currency and floating point number format ting You can find the complete list in the Help file or documentation Specific Windows Types The predefined data types we have seen so far are part of the Pascal lan guage Delphi and FreePascal also include other data types defined by Windows These data types are not an integral part of the language but they are part of the Windows libraries Windows types include new common types such as DWORD or UINT many records or structures several pointer types and so on Among Windows data types the most important type is represented by han dles discussed in Chapter 9 Typecasting and Type Conversions As we have seen you cannot assign a variable to
123. rnal declaration in fact we can specify that our function refers to a function of a DLL that originally had a different name 48 In the 16 bit version of Delphi the external declaration used the name of the library without the extension and was followed by the name directive as in the code above or by an alternative index directive followed by the ordinal number of the function inside the DLL The change reflects a system change in the way libraries were accessed Although Win32 allows access to DLL functions by number Microsoft has stated this won t be supported in the future Notice also that the Windows unit replaced the WinProcs and WinTypes units of the 16 bit version of Delphi Marco Cantu Essential Pascal 108 Chapter 9 Windows Programming A Windows Callback Function We ve seen in Chapter 6 that Pascal supports procedural types A common use of procedural types is to provide callback functions to a Windows API function First of all what is a callback function The idea is that some API functions perform a given action over a number of internal elements of the system such as all of the windows of a certain kind Such a function also called an enumeration function requires the action to be performed on each of the elements as a parameter which is passed as a function or procedure compat ible with a given procedural type Windows uses callback functions in other circumstances but we ll limit our study to this simple
124. rs also makes the code easier for people to read Truly Pascal picked two different operators than C and syntactic derivatives like Java C JavaScript which uses for assignment and for equality test The two elements of an assignment are often called rvalue and lvalue for right value the variable or memory location you are assigning from and left value the value of the expressions being assigned The result of the expression is generally copied to the variable When you copy a record or an array for example the entire data structure is copied to a new memory location an operation that might be time consuming As we ll see in Chapter 7 strings are managed in a different way Conditional Statements A conditional statement is used to execute either one of the statements it contains or none of them depending on a specified test There are two basic flavors of conditional statements if statements and case statements If Statements The i f statement can be used to execute a statement only if a certain condi tion is met if then or to choose between two different alternatives i f then else The condition is described with a Boolean expression A simple Pascal example will demonstrate how to write conditional state ments In this program we ll ask the user for input by using the read function with a single character as parameter var aChar Char begin write znter a character readln achar if achar
125. rted constants const zero 0 global variables var Total Integer list of exported functions and procedures procedure MyProc implementation other units we refer to in the implementation uses D E Marco Cant Essential Pascal Chapter 11 Program and Units 121 hidden global variable var PartialTotal Integer all the exported functions must be coded procedure MyProc begin de Code of procedure MyProc end initialization optional initialization part finalization optional clean up code end The uses clause at the beginning of the interface section indicates which other units we need to access in the interface portion of the unit This includes the units that define the data types we refer to in the definition of other data types such as the components used within a form we are defin ing The second uses clause at the beginning of the implementation section indicates more units we need to access only in the implementation code When you need to refer to other units from the code of the routines and methods you should add elements in this second uses clause instead of the first one All the units you refer to must be present in the project directory or in a directory of the search path you can set the search path for a project in the Directories Conditionals page of the project s Options dialog box C programmers should be aware that the uses statement does not corre spond
126. s In Pascal there are several predefined data types which can be divided into three groups ordinal types real types and strings We ll discuss ordinal and real types in the following sections while strings are covered later in this chapter Delphi also includes a non typed data type called variant discussed in Chapter 10 Strangely enough a variant is a type without proper type check ing It was introduced in Delphi 2 to handle Windows OLE Automation but found its way to other areas of the Delphi libraries Ordinal Types Ordinal types are based on the concept of order or sequence Not only can you compare two values to see which is higher but you can also ask for the next or previous values of any value and compute the lowest and highest possible values The three most important predefined ordinal types are Integer Boolean and char character However there are other related types that have the same meaning but a different internal representation and support a different 10 Although this might sound odd resource strings are available also on Kylix 11 I llalso introduce some types defined by the Delphi libraries not predefined by the compiler which can be considered as predefined types for all practical purposes Marco Cant Essential Pascal 34 Chapter 3 Types Variables and Constants range of values The following table lists the ordinal data types used for rep resenting numbers Size
127. s as refer enced in the text The author and publisher has made his best efforts to prepare this book and the con tent is based upon the latest release of software whenever possible The author and publisher make no representation or warranties of any kind with regard to the complete ness or accuracy of the contents herein and accepts no liability of any kind including but not limited to performance merchantability fitness for any particular purpose or any losses or damages of any kind caused or alleged to be caused directly or indirectly from this book Edition for createspace com Revision 2 April 21 2008 Printed copies of this book can be ordered at http www marcocantu com epascal Marco Cantu Essential Pascal Introduction 3 Introduction This book is dedicated to my family Lella Benedetta and Jacopo The first few editions of Mastering Delphi the best selling series of Delphi books that I wrote between 1995 and 2005 provided an introduction to the Pascal language in Delphi Due to space constraints and because many Del phi programmers looked for more advanced information this material was completely omitted in the later editions of the Mastering Delphi series To overcome the absence of this information I started putting together an ebook titled Essential Pascal After several online only editions I m now making the book available in three formats a free HTML based book with ads a paid PDF
128. s 46 TH ande 106 THEN ese eciieecsreucet ihre 61 Times alal 41 Oh 45 Turbo Pascal 12 MV ar RE psi at 115 Marco Cantu Essential Pascal TVarRec 5 trt rapina carini 77 Type compatibility rule 48 Type Variant Open Array Parameters 76 RK 44 UIN NEE 44 TO tano 119 Unnamed types iii 48 Upp ercasens ottiene 17 User defined data types 48 USES 121 Valiani iaia 45 Diu 73 bist 113 Variant record igri sonan 55 buem d en 105 While neri 65 White Space 18 Wide Char eerte tete 35 WideString iii 90 Windows API i 105 Windows TypesS iii 44 Wirth GEIN in 12 nmm 67 Wofd siir alii 34 Www marcocantu COom 00 195 Zero based array in 54 operatori iii 56
129. sential Pascal 66 Chapter 5 Statements this reason I had to reverse the condition in the code above to obtain a simi lar effect 34 Examples of Loops To explore the details of loops let s look at a small Pascal example The LoopsTest program highlights the difference between a loop with a fixed counter and a loop with an almost random counter The first loop displays a number of strings var I Integer begin for I 1 to 20 do writeln String IntToStr I1 end The second fragment is slightly more complex In this case there is a whi le loop based on a counter which is increased randomly To accomplish this I ve called the Randomi ze procedure which resets the random number gen erator and the Random function with a range value of 100 The result of this function is a number between o and 99 chosen randomly The series of ran dom numbers control how many times the while loop is executed var I Integer begin Randomize I 0 while I lt 1000 do begin I I Random 100 writeln Random Number IntToStr 1 end end Each time you run the program the numbers are different because they depend on the random number generator The following is the output of two separate executions in parallel Random Number 25 Random Number 82 Random Number 68 Random Number 130 Random Number 131 Random Number 140 34 This property is formally known as the De Morgan s laws found herehttp
130. ss and millisec onds hhh in the format string As an alternative you can use the Windows API s GetTickCount function which returns a very precise indi cation of the milliseconds elapsed since the operating system was started Marco Cant Essential Pascal Chapter 10 Variants 117 In this example the speed difference is actually so great that you ll notice it even without a precise timing Variants 00 922 seconds Integers 00 005 seconds The actual values depend on the computer you use to run this program but the relative difference won t change much Summary Variants are so different from traditional Pascal data types that I ve decided to cover them in this short separate chapter Although their primary role is in OLE programming they can be handy to write quick and dirty programs without having even to think about data types As we have seen this has a severe performance penalty Now that we have covered most of the language features let me discuss the overall structure of a program and the modularization offered by units Marco Cantu Essential Pascal 118 Chapter 10 Variants Marco Cantu Essential Pascal Chapter 11 Program and Units 119 Chapter 11 Program And Units Pascal applications make extensive use of units or program modules Units in fact were the basis of the modularity in the language before classes were introduced In a Delphi application every form has a corresponding unit behind
131. step is usually to call Reset to open the file for reading at the beginning Rewrite to open or create it for writing and Append to add new items to the end of the file without removing the older items Once the input or output operations are done you should call CloseFile As an example look at the following code the IntegersToFile demo which simply saves some numbers to a file type IntFile file of Integer var IntFilel IntFile n Integer begin AssignFile IntFilel test my Rewrite IntFilel nie 1 Write IntrFilel n n x 2 write IntFi lel n CloseFile IntFilel end The CloseFi le operation should typically be done inside a finally block to avoid leaving the file open in case the file handling code generates an exception Actually file based operations generate exceptions or not depend ing on the I compiler settings In case the system doesn t raise exceptions you can check the standard IOResult global variable to see if anything went wrong res IOResult if res 0 then writeln 77e test my created correctly else begin write F77e test my creation failed with error writeln res end Marco Cantu Essential Pascal Chapter 12 Files in the Pascal Language 129 There are two rules to consider in the code snippet above The first is that you don t need to call IOResult for every output operation After a failure following output calls will be ignored The second is that the
132. t libraries tend to do the same If you need to work on an array you can always test its bounds by using the standard Low and Hi gh functions which return the lower and upper bounds Using Low and High when operating on an array is highly recommended especially in loops since it makes the code independent of the range of the array Later you can change the declared range of the array indexes and the code that uses Low and Hi gh will still work If you write a loop hard coding the range of an array you ll have to update the code of the loop when the array size changes Low and Hi gh make your code easier to maintain and more reliable Pascal uses arrays mainly in the form of array properties I ll show you some more examples of array properties in the next chapter when discussing Pas cal loops Delphi 4 introduced dynamic arrays arrays that can be resized at runtime allocating the proper amount of memory into Object Pascal Using dynamic arrays is easy but I felt it better to cover them later in the discussion of how Pascal handles memory in Chapter 8 Record Types Record types define fixed collections of items of different types Each ele ment or field has its own type The definition of a record type lists all these fields giving each a name you ll use later to access it Here is a small listing with the definition of a record type the declaration of a variable of that type and few statements using this variable 24
133. tatement introduces new identifiers in the current scope we might hide existing identifiers or wrongfully access another identifier in the same scope as in the first version of this code fragment Even considering this kind of drawback I suggest you get used to with statements because they can be really very handy and at times even make the code more read able You should however avoid using multiple with statements such as with ListBox1 Buttonl do The code following this would probably be highly unreadable because for each property defined in this block you would need to think about which component it refers to depending on the respective properties and the order of the components in the wi th statement Speaking of readability Pascal has no endif or endcase statement If an if statement has a begin end block then the end of the block marks the end of the statement The case statement instead is always terminated by an end All these end statements often found one after the other can make the code difficult to follow Only by tracing the indentations can you see which statement a particular end refers to A common way to solve this problem Marco Cantu Essential Pascal 70 Chapter 5 Statements and make the code more readable is to add a comment after the end state ment indicating its role as in end if Summary This chapter has described how to code conditional statements and loops Instead of writing l
134. ter 9 Windows Programming 109 var Text string begin SetLength Text 100 GetwindowText Hwnd PChar Text 100 Text PChar Text Skip windows with empty titles if Text lt gt then writeln intToStr Hwnd Text Result True end The program calls the Enumwindows API function passing the GetTitle function as its parameter var EWProc TFNwndEnumProc begin EWProc GetTitle Enumwindows EWProc 0 end I could have called the function without storing the value in a temporary procedural type variable first but I wanted to make clear what is going on in this example The effect looks like the following with dozen of lines I ve omitted 66762 Clamwin 66602 Interwise Push client 66700 SkypeO marco cantu 66696 TrayIconManager 66676 HP ProtectToolsSystemTraywindowInstance 66390 windows Sidebar 66382 Apache Service Monitor 262732 TaskEng Task Scheduler Engine Process 66022 HiddenFaxwindow 66018 BluetoothNotificationAreaIconwindowClass 131128 MMDEVAPI Device window 196902 Battery Meter 131346 TSVNCachewi ndow 133024 EnumTitles CodeGear RAD Studio for Microsoft windows EnumTitles dproj 132950 EssentialPascalv3 odt Openoffice org writer 262650 wintech Italia Srl Inbox 4 Mozilla Firefox 133642 XanaNews 1 18 1 6 65780 Program Manager This is a portion of the list of all the existing main windows running in my system Most of them are hidden wi
135. the file type is not available in Delphi for NET Marco Cant Essential Pascal Chapter 5 Statements 59 Chapter 5 Statements If the data types are one of the foundations of Pascal programming the other are statements In its time this idea was clarified by Nicklaus Wirth s out standing book Algorithms Data Structures Programs published by Prentice Hall in February 1976 a classic book still reprinted and available Statements of the programming language are based on keywords and other elements which allow you to indicate to a compiler a sequence of operations to perform Statements are often enclosed in procedures or functions as we ll see in the next chapter Now we ll just focus on the basic types of com mands you can use to create a program Simple and Compound Statements A Pascal statement is simple when it doesn t contain any other statement Examples of simple statements are assignment statements and procedure calls Simple statements are separated by a semicolon X Y Z assignment Randomize procedure call Marco Cantu Essential Pascal 60 Chapter 5 Statements Usually statements are part of a compound statement bracketed within the words begin and end A compound statement can appear anywhere a sim ple Pascal statement can appear Here is an example begin A D B C A 2 end The semicolon after the last statement of the compound statement that is before the end isn t
136. tible with the TVarRec type elements The TVarRec record has the following structure type TVarRec record case Byte of vtInteger vtBoolean vtchar vtExtended vtstring vtPointer vtPChar vtobject vtclass vtwidechar vtPWidechar vtAnsiString vtCurrency vtVariant vtInterface end VInteger Integer VType Byte VBoolean Boolean vchar Char VExtended PExtended vstring PShortString VPointer Pointer vPChar PChar vobject TObject vclass TClass vwidechar widechar vPwideChar Pwidechar vAnsiString Pointer vCurrency PCurrency Vvariant PVariant vInterface Pointer Each possible record has the VType field although this is not easy to see at first because it is declared only once along with the actual Integer size data generally a reference or a pointer Using this information we can actually write a function capable of operating on different data types In the SumA11 function example I want to be able to sum values of different types trans forming strings to integers characters to the corresponding ordinal value and adding 1 for True Boolean values The code is based on a case statement and is quite simple although we have to dereference pointers quite often function SumAll const Args array of const Extended var I Integer begin Result 0 for I Low Args to High Args do case Args I VType of vtInteger Result
137. tick with the more efficient var parameters Except for not having an initial value out parameters behave like var parameters Constant Parameters As an alternative to reference parameters you can use a const parameter Since you cannot assign a new value to a constant parameter inside the rou tine the compiler can optimize parameter passing The compiler can choose an approach similar to reference parameters or a const reference in C terms but the behavior will remain similar to value parameters because the original value won t be affected by the routine In fact if you try to compile the following silly code Delphi will issue an error function DoubleThevalue const value Integer Integer begin value value 2 compiler error 37 Pascal objects in fact are invariably passed by value because they are references themselves For this reason passing an object by reference makes little sense apart from very special cases because it corresponds to passing a reference to a reference Marco Cant Essential Pascal Chapter 6 Procedures and Functions 75 Result Value end Open Array Parameters Unlike C a Pascal function or procedure always has a fixed number of parameters However there is a way to pass a varying number of parameters to a routine using an open array The basic definition of an open array parameter is that of a typed open array This means you indicate the type of the parameter
138. to the Pascal language In Delphi 3 Borland added to the language the concept of interfaces making a leap forward in the expressiveness of classes and their relationships With Kylix Borland made a further step and opened to Pascal Delphi program mers the Linux operating system even if only in its Intel based incarnation Most of the examples of this book can be executed almost unchanged on Linux With the release of version 7 of Delphi and version 3 of Kylix Borland has formally started to call the Pascal or Object Pascal language the Delphi lan guage So Delphi 7 uses the Delphi language Kylix 3 supports both the Delphi and the C languages and Borland ships a Delphi language com piler for the Microsoft s NET architecture This is mainly a cosmetic and marketing change probably due to the fact that the Pascal language was never popular in the US as it used to be and still is in Europe and other areas of the world Delphi 8 for NET has introduced a new breed of Delphi IDE and added extensive support in the language and libraries for Microsoft s NET archi tecture Delphi 8 released at the end of 2003 marked the most extensive and dramatic set of changes to the Object Pascal Delphi language since the introduction of Delphi 1 in 1995 changes that were also adopted in the Win32 Delphi compiler At the time of this writing the latest version of Delphi is RAD Studio 2007 which is now produced by CodeGear a Borland subsid
139. trivial now but previously you had to quit the editor return to DOS run the command line compiler write down the error lines open the editor and jump to the error lines Moreover Borland sold Turbo Pascal for 49 dollars where Microsoft s Pascal compiler was sold for a few hundred Turbo Pascal s many years of success contributed to Microsoft eventual dropping its Pascal compiler product You can actually download a copy of the original version of Borland s Turbo Pascal from the Museum section of the CodeGear s Developer Network http dn codegear com museum 3 See the official Wirth biography at http www cs inf ethz ch wirth Marco Cant Essential Pascal Chapter 1 Short History of the Pascal Language 13 Delphi s Pascal After 9 versions of Turbo and Borland Pascal compilers which gradually extended the language into the Object Oriented Programming OOP realm Borland released Delphi in 1995 turning Pascal into a visual programming language Delphi extends the Pascal language in a number of ways including many object oriented extensions which are different from other flavors of Object Pascal including those in the Borland Pascal with Objects compiler the last incarnations of Turbo Pascal With Delphi 2 Borland brought the Pascal compiler to the 32 bit world actually re engineering it to provide a code generator common with the C compiler This brought many optimizations previously found only in C C compilers
140. ts Most of what these loops do will be familiar if you ve used other programming lan guages so I ll only cover them briefly Marco Cantu Essential Pascal 64 Chapter 5 Statements The For Loop The for loop in Pascal is strictly based on a counter which can be either increased or decreased each time the loop is executed Here is a simple example of a for loop used to add the first ten numbers var total I Integer begin total 0 for I 1 to 10 do total total I This same for statement could have been written using a reverse counter var total I Integer begin total 0 for I 10 downto 1 do total total I The for loop in Pascal is less flexible than in other languages it is not possi ble to specify an increment different than one but it is simple and easy to understand Ifyou want to test for a more complex condition or to provide a customized counter you need to use a while or repeat statement instead of a for loop The counter of a for loop doesn t need to be a number It can be a value of any ordinal type such as a character or an enumerated type Here is an example with characters var aChar Char begin for achar a to z do begin write achar write end Here is another example with an enumeration type TSuit Club Diamond Heart Spade Marco Cant Essential Pascal Chapter 5 Statements 65 var aSuit TSuit begin for asuit Clu
141. tself An example is the simple Beep procedure If you create a new Delphi program add a button and write the following code procedure TForml ButtonlClick sender TObject begin Beep end then as soon as you press the button you ll hear a short sound Now move to the uses statement of the unit and change the code from this uses windows Messages SysUtils Classes to this very similar version simply moving the SysUtils unit before the Win dows unit uses SysUtils Windows Messages Classes Marco Cantu Essential Pascal 124 Chapter 11 Program and Units If you now try to recompile this code you ll get a compiler error Not enough actual parameters The problem is that the Windows unit defines another Beep function with two parameters Stated more generally what happens in the definitions of the first unit you include in the uses statement might be hidden by corresponding definitions of later units The safe solu tion is actually quite simple procedure TForml ButtonlClick sender TObject begin SysUtils Beep end This code will compile regardless of the order of the units in the uses state ments There are few other name clashes in Delphi simply because Delphi code is generally hosted by methods of classes Having two methods with the same name in two different classes doesn t create any problem Units and Programs A Delphi application consists of two kinds of source code files one or more units
142. u cannot insert a space in an identifier every first letter of a word should be capitalized MyLongIdentifier MyVeryLongAndAlmoststupidIdentifier This is often called Pascal casing to contrast it with the so called Camel casing of Java and C derived languages which capitalizes internal words but requires an initial lowercase letter like in myLongIdentifier White Space Other elements completely ignored by the compiler are the spaces new lines and tabs you add to the source code All these elements are collectively known as white space White space is used only to improve code readability it does not affect the compilation in any way Unlike traditional BASIC Pascal allows you to write a statement on several lines of code splitting a long instruction on two or more lines The drawback at least for many BASIC programmers of allowing statements on more than one line is that you have to remember to add a semicolon to indicate the end 6 InDelphithere is only one exception to the case insensitive rule of Pascal the Register procedure of a components package must start with the uppercase R because of a C Builder compatibility issue Of course when you refer to identifiers exported by other languages like a native Win32 function or a NET class you might have to use the proper capitalization Marco Cant Essential Pascal Chapter 2 Coding in Pascal 19 of a statement or more precisely to separate a statem
143. ven erre 128 ee laica 52 Assignable typed constants 32 LT cirie 57 AssignFile 44 4 ilari 128 Assignment eere erbe ith eee 60 aleng 130 Banker s Rounding 46 hr SN 123 Blog ena eet tree reote innen 6 Boole atin cist aiT evar 33 35 Br ale agonismo 67 ISATA R EN E E A E TTE 34 Callback functions 108 Calling Conventions ii 78 Camel casing iii 18 Cardinal a nandan 34 Carelli 56 CAS Gis licia tai 63 Castind clp i EA a 45 Ceana tn ati 79 6 T geen ec ER Characters aeternae 35 Chissa nni pe 35 45 Ghrome ect ian 3 14 Index 137 Close Filen s ac vto testi dte 128 Cap eret re eps 110 Colon equal operator 60 Command line parameters 131 Comments iere ene 16 COMPD rara 40 Compiler directives e 17 Conditional agi doac 61 E eT EE 74 Constant characters eese 35 E eT EE 31 Goin cH 67 Copyright iere erre te 4 HES esce ete rete eee DER asi 40 Data l ypes ee ee PORE 33 Dale nens m D PS 41 DateTimeTOoStr sees 42 DayOfWeek esee eene tentent 42 Decisi rali 38 Decodeate sss sss 42 RT a Ut eene redeem See dee 85 Delphi nta 3 13 Delphi 2007 Handbook 135 Dereferencing iii 57 DISPOSE irre etre aene ve aaie een 56 row LER 39 DOT 64 DPR rtt eren 1
144. y You can check them out in the FreePascal and GNU Pascal documenta tion So I m actually going to concentrate on Windows and Delphi in this chapter Windows Handles Among the data types introduced by Windows in Delphi handles represent the most important group The name of this data type is THandle and the type is defined in the Windows unit as type THandle Longword Handle data types are implemented as numbers but they are not used as such In Windows a handle is a reference to an internal data structure of the system For example when you work with a window or a form in Delphi terms the system gives you a handle to the window The system informs you that the window you are working with is window number 142 for exam ple From that point on your application can ask the system to operate on window number 142 moving it resizing it reducing it to an icon and so on Many Windows API functions in fact have a handle as the first parameter This doesn t apply only to functions operating on windows other Windows API functions have as their first parameter a GDI handle a menu handle an instance handle a bitmap handle or one of the many other handle types In other words a handle is an internal code a sort of ID you can use to refer to a specific element handled by the system including a window a bitmap an icon a memory block a cursor a font a menu and so on In Del phi you seldom need to use handles directly s
145. yTemp1 vari able of the TDayTemperatures type as follows type TDayTemperatures array 1 24 of Integer var DayTempl TDayTemperatures procedure AssignTemp begin DayTempl 1 54 DayTempl 2 52 DayTempl 24 66 DayTempl 25 67 compile time error An array can have more than one dimension as in the following examples type TMonthTemps array 1 24 1 31 of Integer TYearTemps array 1 24 1 31 Jan Dec of Integer These two array types are built on the same core types So you could also declare them using the preceding data types as in the following code type TMonthTemps array 1 31 of TDayTemperatures TYearTemps array Jan Dec of TMonthTemps This declaration inverts the order of the indexes as presented above but it also allows assignment of whole blocks between variables For example the following statement copies January s temperatures to February var ThisYear TYearTemps begin ThisYear Feb ThisYear Jan Marco Cantu Essential Pascal 54 Chapter 4 User Defined Data Types You can also define a zero based array an array type with the lower bound set to zero Generally the use of more logical bounds is an advantage since you don t need to use the index 2 to access the third item and so on Win dows Linux and Mac OS X however invariably uses zero based arrays because they are based on the C language and for this reason Delphi and many other componen
146. ype which indicates untyped pointers such as voi d in the C language If you use an untyped pointer you should use GetMem instead of New The GetMem procedure is required each time the size of the memory variable to allocate is not defined The fact that pointers are seldom necessary in Pascal is an interesting advan tage of this environment Nonetheless understanding pointers is important for advanced programming and for a full understanding of Delphi object model which use pointers behind the scenes File Types Another Pascal specific type constructor is the file type File types represent physical disk files certainly a peculiarity of the Pascal language You can define a new file data type as follows type IntFile file of Integer Then you can open a physical file associated with this structure and write integer values to it or read the current values from the file The use of files in Pascal is quite straightforward but in Delphi there are also some components that are capable of storing or loading their contents to or from a file There is some serialization support in the form of streams and there is also database support Conclusion This chapter discussing user defined data types completes our coverage of the Pascal type system Now we are ready to look into the statements the lan guage provides to operate on the variables we ve defined 30 Files based examples are covered in Chapter 12 Notice however that

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