[fpc-pascal] inherited interfaces not seen by queryinterface / supports

[Graeme Geldenhuys]

On 2016-10-27 02:13, David Emerson wrote:
>    t_2 = class (_obj_type_, i_2)
>      // i_2 inherits from i_1, compiler requires i_1 implementation
>      procedure do_one;
>      procedure do_two;
>      end;


A common misconception about how interfaces work. In fact, I don't
actually know why FPC and Delphi bother with Interface Inheritance,
because I simply don't see the point.

To make your "t_2" class support both interface, you need to specify
both in the class declaration. Even though i_2 inherits from i_1, both
i_1 and i_2 must be specified in the t_2 class.

Again, I don't know why it is done like this, and works very different
to class inheritance. Delphi 7 works exactly the same in this regard.
Maybe somebody with more knowledge on the subject could shed some light.

Here is a D7 example:

=======================================
program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  IOne = interface
  ['{32271879-1C00-4A02-A9C0-6948844028D6}']
    procedure One;
  end;

  ITwo = interface(IOne)
  ['{F3027177-6638-4984-B842-7D4E70299056}']
    procedure Two;
  end;

  {  If you only specify ITwo here, then the second Supports() call
     for i1.One will never execute. }
  TMyObject = class(TInterfacedObject, IOne, ITwo)
  private
    // IOne interface
    procedure One;
    // ITwo interface
    procedure Two;
  end;

procedure TMyObject.One;
begin
  writeln('One');
end;

procedure TMyObject.Two;
begin
  writeln('Two');
end;

var
  o: TMyObject;
  i1: IOne;
  i2: ITwo;
begin
  o := TMyObject.Create;
  if Supports(o, ITwo, i2) then
    i2.Two;
  if Supports(o, IOne, i1) then
    i1.One;
  i2 := nil;
  i1 := nil;

end.

=======================================


Regards,
  Graeme

-- 
fpGUI Toolkit - a cross-platform GUI toolkit using Free Pascal
http://fpgui.sourceforge.net/

My public PGP key:  http://tinyurl.com/graeme-pgp