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:
var
AllocI: ID3DXAllocateHierarchy_Interface;
begin
AllocI:= pAlloc;
// Now patch interface VMT - so it will point to truncated interface (without IUnknown)
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 end;
But I really want FPC be compatible with Delphi in this situation.
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