delphi: you can only access protected identifiers from parent classes in your own “Self” scope, or when you are “friends” with your parent (so you are in the same unit)

An interesting question from a while back: [WayBack] delphi – Should a descendant class’ method’s variable that is identical to Self, have access to its ancestor’s protected methods? – Stack Overflow

In unit A:

TParent = class
protected
  function DoSomething: TParent;
end;

In unit B:

TChild = class(TParent)
public
  procedure DoAnotherThing;
end;

implementation

procedure TChild.DoAnotherThing;
begin
  DoSomething.DoSomething
end;

…

This won’t compile, throwing a

cannot access protected symbol TParent.DoSomething

The kicker here is that the error message makes you think you are operating in Self context, but you are not as you are calling DoSomething.DoSomething where only the first DoSomething is in your Self context, but the second .DoSomethingis in the context of any TParent instance trying to access a public identifier.

Stefan Glienke posted [WayBack] a more elaborate answer explaining some workarounds.

–jeroen