The Delphi approach

In the Delphi approach to Object Oriented Programming, everything revolves around the concept of 'Classes'. A class can be seen as a pointer to an object, or a pointer to a record.

The prototype declaration of a class is as follows :

TObj = Class [(ParentClassType)]
  [Constructor ConstructorName;]
  [Destructor DestructorName;]
  Field1 : Type1;
  ...
  Fieldn : Typen;
  Method1;
  Method2;
  [private
  PrField1 : PrType1;
  ...
  PrFieldn : PrTypen;
  PrMethod1;
  ...
  PrMethodn;]
  [public
  PuField1 : PuType1;
  ..
  Pufield1 : PuTypen;
  PuMethod1;
  ...
  PuMethodn;]
  end;

As you can see, the declaration of a class is almost identical to the declaration of an object. The real difference between objects and classes is in the way they are created;

Classes must be created using their constructor. Remember that A class is a pointer to an object, so when you declare a variable of some class, the compiler just allocates a pointer, not the entire object. The constructor of a class returns a pointer to an initialized instance of the object.

So, to initialize an instance of some class, you do the following :

  ClassVar:=ClassType.ConstructorName;

Remark : Free Pascal doesn't support the concept of properties yet.