Question

I have a main class and several inherited classes that implement a method with the same name, like this:

MainClass = class(TImage)
  //main class methods...
end;

MyClass1 = class(MainClass)
  procedure DoSomething;
end;

MyClass2 = class(MainClass)
  procedure DoSomething;
end;

MyClass3 = class(MainClass)
  procedure DoSomething;
end;

I also have a TList containing pointers to object instances (of several classes).
If I want to call the right DoSomething procedure for each class, do I use the following?

if TList[i] is MyClass1 then
  MyClass1(TList[i]).DoSomething
else if TList[i] is MyClass2 then
  MyClass2(TList[i]).DoSomething
else if TList[i] is MyClass3 then
  MyClass3(TList[i]).DoSomething

Is there some casting method that allows me to do this in a few lines of code?

Answer 1

Yes, virtual polymorphism 🙂

MainClass = class(TImage)
  procedure DoSomething; virtual;
end;

MyClass1 = class(MainClass)
  procedure DoSomething; override;
end;

MyClass2 = class(MainClass)
  procedure DoSomething; override;
end;

MyClass3 = class(MainClass)
  procedure DoSomething; override;
end;

And then just:

if TList[i] is MainClass then
  MainClass(TList[i]).DoSomething

If you don’t want to do an empty MainClass.DoSomething procedure, you can also mark it virtual; abstract;.