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What are classes in Pascal?

Dian Us Suqlain

Every object-oriented programming (OOP) language involves concepts of objects, classes, inheritance, polymorphism, etc.

Pascal is a strongly typed object-oriented language and uses classes to perform various functionalities. In Pascal, classes are allocated on the heap.

Characteristics of classes in pascal

A class defines the structure of a program. They are basically the blueprint of your program and includes characteristics such as:

  • constructors
  • variables
  • fields
  • properties
  • methods
  • instances or objects, etc.

A class is defined similar to the way an object is specified i.e., using the type keyword.

We implement classes in a program to apply proper object-oriented behavior, as objects in Pascal do not take part in polymorphismthe ability of an object to take on many forms.

A single class can include multiple characteristics

Simple class in pascal

The following code snippet shows the basics of how to declare a class and its attributes such as constructors, procedures, function, etc.

type class-name = class  
   private
      field1 : integer;  
      field2 : string;  
      field2 : boolean;  
            ...
   
   public
      constructor create();
      procedure proc1;  
      function func1(): function-type;
      function func2(param1, param2, integer): integer; overload;
end;  
var classvar : class-name;

private and public are the type or scope modifiers of a class that increases or decreases code visibility to the user.

create is a pre-defined constructor in Pascal that helps us create constructors for a class. There can be more than one constructor in a class, while only one destructor is allowed for each class.

private and public keywords

As described above, these keywords are scope modifiers of a class type. The main difference between them is:

We cannot read or write from another module if it is declared as a private field or property.

A private member is invisible outside of the unit or program where its class is declared.

If a member is declared public, it is visible wherever its class can be referenced.

We can read or write from another module if it is declared as a public field or property.

Abstract classes in Pascal

Pascal supports the concept of abstraction, which allows hiding the implementation details and showing only functionality to the user. Abstract classes are only inherited from other classes and not explicitly instantiated.

We can create an abstract class in Pascal the same way other classes are defined. However, we add the abstract keyword before the class name to make it abstract.

type class-name = abstract class (Root)
      procedure proc1; abstract;
      function func1(): function-type;
               ...
   end;

Note: A class in Pascal can be declared abstract even if it does not contain any abstract virtual methodsA method is a virtual or dynamic method with no implementation in the class where it is declared. We should write its implementation to a descending class.

Sealed classes in Pascal

Pascal has introduced a new concept, i.e., Sealed Classes.

sealed is a keyword in Pascal that makes a class unable to support inheritance. By making a class, sealed means that it cannot extend through inheritance.

type class-name = class sealed
      procedure proc1;
      function func1(): function-type;
      other memberList;
               ...
   end;

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