Question

For example, is there a way to find out that this class has a virtual constructor (at runtime)?

   TMyClass = class(TObject)
     MyStrings: TStrings;
     constructor Create; virtual;   
   end;

For example, in this code I would like to test if the class referenced by Clazz has a virtual constructor:

procedure Test;
var
  Clazz: TClass;
  Instance: TObject;
begin
  Clazz := TMyClass;
  Instance := Clazz.Create;
end;

Is there a simple solution, for example using RTTI, which works in Delphi 6 to 2009?

Answer 1

Looking through the TypInfo unit, it doesn’t look like there’s any way to tell if a method is virtual using RTTI or not. If you have a class reference, though, you can probably roll your own method by examining the VMT.

According to Allen Bauer, in an answer to this question, you can find the end of the VMT immediately before the value pointed to by vmtClassName. The first user-defined virtual method (if any) is found at the address of the class reference. In other words, pointer(Clazz)^. Now that you know the start and end points of the user-defined section of the VMT, it shouldn’t be too difficult to make a while loop that compares each pointer in the table against the Code section of a method pointer to Clazz.create casted to a TMethod. If you get a match, then it’s a virtual method. If not, then it isn’t.

Yes, it’s a bit of a hack, but it’ll work. If anyone can find a better solution, more power to them.