How to overload a Delphi class function

Question

I have a Delphi (2007) class function like this:

Class Function InitGlobal : TForm; Overload; Virtual; Abstract;

in some descendant class I try to:

Class Function InitGlobal : TDescendentForm; Overload; Override;

But Delphi complains that TDescendentForm.InitGlobal differs from the previous declaration (despite the pressence of the "Overload" directive).

I guess that function result types can not be overloaded. Which is the correct way of define such overloading if any?

I checked Function overloading by return type?, but it mentions the cons and pros of making such overloading with no mention to Delphi.

In my humble opinion, you either overload or override, but you can't do both simultaneously. An overload needs to have a different method signature, while an override indicates that the method shall replace the method of the same name from your derived type. Perhaps a more detailed code sample could help further. — Will Marcouiller
Your opinion is irrelevant, @Will. The fact is that you can do both simultaneously. It happens several times in the RTL and VCL. — Rob Kennedy
Thanks for the info, @RobKennedy. I have never done both simultaneously in my career, since I have always expected the overloads to be methods which only define different parameters for a given behaviour, and the overrides to be methods which only redefine the behaviour of an object. I have never happened to do both, and I would tend to think of it as a design flaw, somehow. But hey, if you say it is done multiple times in the RTL and VCL, it must be good for some reason that I simply don't need. =) — Will Marcouiller

David Heffernan David Heffernan · Accepted Answer · 2014-07-14T19:08:25

Function overloading can only be based on parameters, and not on return values. There is no way for you to have two overloads that differ only in their return value.

What's more, even if you could do that, you are attempting to use override, and change the function's signature. That is also impossible. An override must have an identical signature to the function being overridden.

What you can do is override the function, and keep the same signature. You can remove the overload directive. Then your derived function would look like this:

class function InitGlobal: TForm; override;

Now, there is nothing to stop you returning an instance of TDescendentForm. That is still a TForm. What you would be doing here is returning a more a derived type at runtime.

The implementation might look like this:

Result := TDescendentForm.Create(...);
// compile time type of Result is TForm, but that is assignment compatible
// at run time with an instance of TDescendentForm

How to overload a Delphi class function

1 Answers