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Classes This section shows the classes under Pascal programming languages: CHAPTER 26 You have seen that Pascal Objects exhibit some characteristics of object-oriented paradigm. They implement encapsulation, data hiding and inheritance, but they also have limitations. For example, Pascal Objects do not take part in polymorphism. So, classes are widely used to implement proper object-oriented behavior in a program, especially the GUI-based software. A Class is defined in almost the same way as an Object, but is a pointer to an Object rather than the Object itself. Technically, this means that the Class is allocated on the Heap of a program, whereas the Object is allocated on the Stack. In other words, when you declare a variable the object type, it will take up as much space on the stack as the size of the object, but when you declare a variable of the class type, it will always take the size of a pointer on the stack. The actual class data will be on the heap. Defining Pascal Classes: A class is declared in the same way as an object, using the type declaration. The general form of a class declaration is as follows: type class-identifier = class private field1 : field-type; field2 : field-type; ... public constructor create(); procedure proc1; function f1(): function-type; end; var classvar : class-identifier; TUTORIALS POINT Simply Easy Learning Page 152
It’s worth to note following important points: Class definitions should come under the type declaration part of the program only. A class is defined using the class keyword. Fields are data items that exist in each instance of the class. Methods are declared within the definition of a class. There is a predefined constructor called Create in the Root class. Every abstract class and every concrete class is a descendant of Root, so all classes have at least one constructor. There is a predefined destructor called Destroy in the Root class. Every abstract class and every concrete class is a descendant of Root, so, all classes have at least one destructor. Let us define a Rectangle class that has two integer type data members - length and width and some member functions to manipulate these data members and a procedure to draw the rectangle. type Rectangle = class private length, width: integer; public constructor create(l, w: integer); procedure setlength(l: integer); function getlength(): integer; procedure setwidth(w: integer); function getwidth(): integer; procedure draw; end; Let us write a complete program that would create an instance of a rectangle class and draw the rectangle. This is the same example we used while discussing Pascal Objects. You will find both programs are almost same, with the following exceptions: You will need to include the {$mode objfpc} directive for using the classes. You will need to include the {$m+} directive for using constructors. Class instantiation is different than object instantiation. Only declaring the variable does not create space for the instance, you will use the constructor create to allocate memory. Here is the complete example: TUTORIALS POINT Simply Easy Learning Page 153
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Pascal Tutorial
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ABOUT THE TUTORIAL Pascal Tutorial
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Declaring Constants ...............
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Accessing Two-Dimensional Array Ele
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CHAPTER 1 Pascal Overview This chap
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CHAPTER 2 Environment This section
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Name and version of text editor can
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Pascal Hello World Example Followin
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CHAPTER 4 Basic Syntax You have see
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The Pascal character set consists o
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Type Declarations: The type declara
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Subrange Types Subrange types allow
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Pascal programming language also al
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Enumerated Variables You have seen
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CHAPTER 7 Constants This section sh
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CHAPTER 8 Operators This section sh
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Checks if the value of left operand
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When the above code is compiled and
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program beBitwise; var a, b, c: int
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CHAPTER 9 Decision Making This sect
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Example: Let us try a complete exam
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Example: Let us try a complete exam
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Nested if-then Statements It is alw
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The following rules apply to a case
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Flow Diagram: Example: The followin
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CHAPTER 10 Loops This section shows
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Flow Diagram: Here, key point of th
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Flow Diagram Example: program forLo
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Example: program repeatUntilLoop; v
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Example: program exBreak; var a: in
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Example: program exContinue; var a:
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Example: The following program illu
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Defining a Function: In Pascal, a f
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variable named m. The variable m is
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program exRecursion; var num, f: in
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procedure swap(x, y: integer); var
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Now, let us extend the program litt
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Let us change the previous example
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12 function AnsiStrLower( Str: PCha
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CHAPTER 15 Boolean This section sho
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CHAPTER 16 Arrays This section show
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Initializing Arrays In Pascal, arra
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Two-Dimensional Arrays: The simples
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The following example declares and
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Passing Arrays as Subprogram Argume
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Following are some valid pointer de
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To check for a nil pointer, you can
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program exPointers; const MAX = 3;
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program exPointers; const MAX = 3;
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Value of num = 3000 Value available
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Page 119: Set Operators You can perform the f
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Page 128 and 129: Reads data from a file into memory
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Page 132 and 133: CHAPTER 22 Memory Management This s
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Page 138 and 139: Returns segment 37 procedure SetMem
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Page 164 and 165: Constructors and Destructors for Pa
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