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delphidelphi-10.2-tokyo

Delphi virtual constructor

I was reading this article because I wanted to understand the usefulness of class of [ClassName] and I have seen that they declare a virtual constructor. So I have made a test that you can see here:

enter image description here

I understand (from that article) that virtual constructors are useful when I don't know at compile-time the class I want to construct and I can use the class of. In the code I have shown above, what is the difference?

If I declared TFirst constructor as virtual without overriding TSecond I'll get the warning of course ad I can remove that with a reintroduce or override. But isn't the constructor automatically overridden (look at the code on the left)? I think that they are equivalet.

Solution

  • Execute this code with both variants and you will see the difference.

    type
  TFirstClass = class of TFirst;

constructor TFirst.Create;
begin
  Writeln('TFirst.Create');
end;

constructor TSecond.Create;
begin
  Writeln('TSecond.Create');
end;

var
  firstClass: TFirstClass;
  first: TFirst;
begin
  firstClass := TSecond;
  first := firstClass.Create;
end.