[fpc-pascal] using Interfaces in descendant classes 2.6.4 vs 3.x

Graeme Geldenhuys mailinglists at geldenhuys.co.uk
Sat Nov 4 00:09:26 CET 2017


Some of my programs were last compiled with FPC 2.6.4, and there I used 
Corba-style Interfaces without a problem. I've now recompiled those 
projects with FPC 3.0.4 and the program immediately crashes.

Debugging I found that I was trying to use a interface reference, but 
forgot to check if it was nil or not. I've fixed the code, but the issue 
is still not resolved. The class in question DOES implement the 
interface, but FPC 3.x always returns nil, whereas FPC 2.6.4 correctly 
returns a Interface reference.

The class hierarchy is as follows...

      TBaseApplication = class(TObject, ICmdLineParams);
      TX11Application = class(TBaseApplication)
      TfpgApplication = class(TX11Application)

In my applications I only work with a TfpgApplication instance. If the 
interface is fully implemented in TBaseApplication, why does FPC 3.x 
_always_ report that TfpgApplication doesn't support the ICmdLineParams?

My code looks as follows:

   cmd: ICmdLineParams;
   if Supports(fpgApplication, ICmdLineParams, cmd) then
     if cmd.HasOption('n', 'newinstance') then
     else if not AnotherInstance then
      // ICmdLineParams interface is not supported

I looked at the wiki and the FPC 3.x "User Changes" pages and the only 
reference to Interface changes is this.


The wiki is very vague with what it means. Either way, I don't consider 
my code wrong. I'm using a class hierarchy and the base class does 
implement the Interface in question fully, so why does FPC 3.x think it 
doesn't? Bottom line, is this a FPC bug and how to I change my code to 
work until the bug can be fixed?


fpGUI Toolkit - a cross-platform GUI toolkit using Free Pascal

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