Question

I have CObject as main class and CRock, CDesk, CComputer as derivates from CObject. I would like to write a function that reads a class enumeration (integer probably like OBJECT_COMPUTER) and returns the specific type.

Example:

function createObject( iType : Integer ) : CObject;
begin
  case iType of
    OBJECT_ROCK : Result := CRock.Create();
    OBJECT_DESK : Result := CDesk.Create(); 
  end;
end;

so I can use it like this: myRock := createObject( OBJECT_ROCK );

Now my problem is that the object returned is the main class parent and I can’t use Rock functions on ‘myRock’ without type casting ‘createObject( OBJECT_ROCK )’ from CObject to CRock and I don’t want to have 3 functions for each sub-class. Any ideas? Thanks in advance.

Answer 1

If I understood correct, you’d declare a skeleton of derived functionality on the base class with abstract methods, then override and implement the method in each derived class.

type
  CObject = class
    procedure DoIt; virtual; abstract;
  end;
  CRock = class(CObject)
    procedure DoIt; override;
  end;
  CDesk = class(CObject)
    procedure DoIt; override;
  end;


var
  myRock: CObject;
begin
  myRock := createObject(OBJECT_ROCK);
  myRock.DoIt;
  myRock.Free;
end;

In the above example, ‘DoIt’ call on the ‘myRock’ instance would be correctly resolved to the method of that class.

If this is relevant at all read about abstract methods here.