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

[David Emerson]

On 10/27/2016 06:01 AM, Tony Whyman wrote:


That is you can build an object hierarchy to parallel an interface
hierarchy.

I don't use the "Supports" primitive. - so maybe there is a bug in this
feature but otherwise, what is the problem with interface inheritance?


Yes, the problem is with "Supports" and "QueryInterface".
All of the examples you gave work fine (I didn't test them but they look 
like tests I've done)


The situation where "Supports" is needed looks more like this:

function do_something (t : tobject);
begin
  if supports (t, i_my_interface) then 


So the situation is that we were passed an object, and we don't know 
what it is; but if we learn that it supports a particular interface, 
then we want to do something with that interface.


~David.



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

[David Emerson]

On 10/27/2016 05:18 AM, Graeme Geldenhuys wrote:

On 2016-10-27 12:42, Marcos Douglas B. Santos wrote:

It is a "bug by design".


The worst kind!



Actually, I think this is should be corrected in both modes (COM and CORBA).


Indeed, and even if it is fixed in FPC, it shouldn’t break any existing
code either. So it’s a win-win situation.

If nothing else, it definitely should be fixed for CORBA Interfaces, but
I highly recommend to the powers that be, to fix it for both interface
modes.

Regards,
  Graeme



+1

Thanks as always

~David


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

[Tony Whyman]

On 27/10/16 03:47, Graeme Geldenhuys wrote:

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.


I must be missing something here because interface inheritance seems to 
work fine for me e.g.


i1 = interface
 procedure Do1;
end;

i2 = interface(i1)
 procedure Do2;
end;

TMyObject = class(TInterfacedObject,i2)
public
  procedure Do1;
  procedure Do2;
end;

var intf: i2;
begin
  i2 := TMyObject.Create;
  i2.Do1; {seems to work for me}
End;

I have plenty of examples where an interface is inherited and includes 
the inherited methods and properties.


I also have cases where e.g.

var SomeInterface: IUnknown;
begin
   SomeInterface := TMyObject.Create;
end;

that is the inherited interface is extracted from the object. There are 
also useful cases, where e.g.


TMyObject1 = class(TInterfacedObject,i1)
public
  procedure Do1;
end;

TMyObject2 = class(TMyObject1,i2)
public
  procedure Do2;
end;

That is you can build an object hierarchy to parallel an interface 
hierarchy.


I don't use the "Supports" primitive. - so maybe there is a bug in this 
feature but otherwise, what is the problem with interface inheritance?


Tony Whyman
MWA
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Re: [fpc-pascal] inherited interfaces not seen by queryinterface / supports

[Marcos Douglas B. Santos]

On Thu, Oct 27, 2016 at 10:18 AM, Graeme Geldenhuys
 wrote:
>> It is a "bug by design".
>
> The worst kind!

Yeah, you're right.

>> Actually, I think this should be corrected in both modes (COM and CORBA).
>
> Indeed, and even if it is fixed in FPC, it shouldn’t break any existing
> code either. So it’s a win-win situation.

Win-win situation, I think so.

> If nothing else, it definitely should be fixed for CORBA Interfaces, but
> I highly recommend to the powers that be, to fix it for both interface
> modes.

+1


Marcos Douglas
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Re: [fpc-pascal] inherited interfaces not seen by queryinterface / supports

[Graeme Geldenhuys]

On 2016-10-27 12:42, Marcos Douglas B. Santos wrote:
> It is a "bug by design".

The worst kind!


> Actually, I think this is should be corrected in both modes (COM and CORBA).

Indeed, and even if it is fixed in FPC, it shouldn’t break any existing
code either. So it’s a win-win situation.

If nothing else, it definitely should be fixed for CORBA Interfaces, but
I highly recommend to the powers that be, to fix it for both interface
modes.

Regards,
  Graeme

-- 
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http://fpgui.sourceforge.net/

My public PGP key:  http://tinyurl.com/graeme-pgp
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Re: [fpc-pascal] inherited interfaces not seen by queryinterface / supports

[Marcos Douglas B. Santos]

On Thu, Oct 27, 2016 at 1:12 AM, Graeme Geldenhuys
 wrote:
> In the Stack Overflow link I posted - further down the page - a person
> named Jasper mentions that in is QueryInterface implementation bug where
> QueryInterface on checks for valid Interface signatures in the top level
> of the interface hierarchy, and doesn't actually check for inherited
> interfaces.  Anyway, read Jasper's comment to see his exact words.
>
>http://stackoverflow.com/a/39496815
>
>
> If nothing else, surely FPC's CORBA style interfaces should not
> implement the COM style interface bug?

It is a "bug by design". But, I agree with you.
Actually, I think this is should be corrected in both modes (COM and CORBA).

Best regards,
Marcos Douglas
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Re: [fpc-pascal] inherited interfaces not seen by queryinterface / supports

[Graeme Geldenhuys]

On 2016-10-27 03:47, Graeme Geldenhuys wrote:
> 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.

I did some quick internet searching. According to Danny Thorpe (an ex
Borland employee I believe), the reason Delphi works so "weird"
regarding Interface Inheritance, is because Delphi is very Windows
centric, and initially Delphi Interfaces were COM compatible interfaces.
Microsoft made some screw-up in COM, and Delphi followed suite.

  http://stackoverflow.com/a/4380371


"The reason is because COM and ActiveX allow an implementation to
implement a descendent interface (your IChild) but deny the ancestor of
that interface (IParent). Since Delphi interfaces are intended to be COM
compatible, that's where this goofy artifact comes from." -- Danny Thorpe


Now my question to the FPC developers are... For a long time now,
Interface support does not always mean COM or ActiveX support, so why
still does FPC do the same as Delphi? And even worse, why does this same
behaviour happen in FPC's CORBA style interfaces - which has *nothing*
to do with COM or ActiveX???

Here as a FPC (CORBA style interfaces) example of the problem with
Interface Inheritance.


program Project1;

  {$mode objfpc}{$H+}
  {$interfaces corba}

uses
  SysUtils;

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

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

  TMyObject = class(TObject, ITwo) //  << IOne is not specified
  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  // this is never true
i1.One;
  i2 := nil;
  i1 := nil;
  o.Free;
end.



In the Stack Overflow link I posted - further down the page - a person
named Jasper mentions that in is QueryInterface implementation bug where
QueryInterface on checks for valid Interface signatures in the top level
of the interface hierarchy, and doesn't actually check for inherited
interfaces.  Anyway, read Jasper's comment to see his exact words.

   http://stackoverflow.com/a/39496815


If nothing else, surely FPC's CORBA style interfaces should not
implement the COM style interface bug?

Regards,
  Graeme

-- 
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http://fpgui.sourceforge.net/

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

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