How to inherit class with generic method?

Question

There is a class

TFoo = class
  function GetValue<T>: T;
  procedure SetValue<T>(AValue: T);
end;

The compiler doesn't allow to define methods of this class as virtual. How class

TChildFoo = class(TFoo)

may use methods of ancestor?

Answer 1

As you have observed, generic methods cannot be virtual. This type

type
  TFoo = class
    function Bar<T>: T; virtual;
  end;

is rejected with the compiler with the following error:

E2533 Virtual, dynamic and message methods cannot have type parameters

Your class is declared like this:

type
  TFoo = class
    function GetValue<T>: T;
    procedure SetValue<T>(AValue: T);
  end;

and you ask how a child class can use these methods. For instance, you can do this:

type
  TChildFoo = class(TFoo)
    procedure DoSomething<T>;
  end;

....

procedure TChildFoo.DoSomething<T>;
begin
  SetValue(GetValue<T>);
end;

In other words, you can certainly use these methods in the parent class. You just cannot declare them to be virtual and override them.

If you have a parameterized class (rather than a parameterized method) then you can declare virtual methods, and override them.

type
  TFoo<T> = class
    function Bar: T; virtual;
  end;

  TChildFoo<T> = class(TFoo<T>)
    function Bar: T; override;
  end;

So, these are your options. As to how to solve your problem, that depends very much on what the problem is.