[fpc-devel] VMT compatibility

Alexey Barkovoy clootie at ixbt.com
Sun Jan 30 11:35:53 CET 2005

----- Original Message ----- 
From: "Florian Klaempfl" <F.Klaempfl at gmx.de>

>> For example "DECLARE_INTERFACE_(IClassFactory, IUnknown)" in C++ is expanded 
>> to "struct FAR IClassFactory : public IUnknown", so really public parts of 
>> VMT (with positive offset) are always [should be] compatible. Surely we can't 
>> talk about FULL compatibility of classes, but exposing "interfaces" to other 
>> languages should be possible.
> Use raw interfaces for such a purpose.

Problem is here:

>> Problem here is what I'm forced to use some these COM incompatibe 
>> "interfaces" exposed by MS programmers in DirectX API. These interfaces look 
>> like real COM interfaces but without IUnknown embedded. In Delphi I can use 
>> them just by creating abstract class just like in C++, but in FPC it's 
>> impossible to do so. And I've seen other reports when people were forced to 
>> use similiar cripped interfaces exposed by C++ programmers. So it's the real 
>> problem not just artificial one.

Delphi and FreePascal simplest interfaces already includes IUnknown. So using 
raw interfaces is incompatible here too.
Actually I have working solution and it's based as you hinted on interface 
feature of object pascal, but after quering interface a have to patch object 
instance so pointer to VMT start pointing above IUnknown part of it. Something 
like this:

  AllocI: ID3DXAllocateHierarchy_Interface;
  AllocI:= pAlloc;
  // Now patch interface VMT - so it will point to truncated interface (without 
  Inc(PInteger(AllocI)^, $C);

  // Call C++ function and pass to it our "special" interface
  Result := D3DXLoadMeshHierarchyFromX_FPC_{$}(Filename, MeshOptions, 
pD3DDevice, AllocI, pUserDataLoader, ppFrameHierarchy, ppAnimController);

  Dec(PInteger(AllocI)^, $C); // Patch interface VMT - BACK

But I really want FPC be compatible with Delphi in this situation.

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