[holy_city]

This is similar to the work done in winapi [1], com-imp[2] and my own tangential work on porting VST3 [3] to Rust [4]

I'm really glad MS is doing this. What needs to be a bit clearer to me is how they maintain ABI compatibility under the hood of MSVC for COM interfaces (which uses the vtable layout of an inherited class) and how that's compatible with MinGW/GCC stacks on Windows, mostly what can break it. I got stuck porting VST3 with multiple inheritance, and it was a headache trying to reverse engineer the appropriate struct layouts for COM implementations.

[1] https://github.com/retep998/winapi-rs/blob/0.3/src/macros.rs

[2] https://github.com/Connicpu/com-impl

[3] https://github.com/steinbergmedia/vst3sdk

[4] https://github.com/m-hilgendorf/cli-host (sorry for the messy code, it was a weekend of hacking away trying to host a VST3 in pure Rust)

[barrkel]

COM is an ABI standard. The structs are defined in terms of C with Winapi (stdcall) calling convention. Very little needs reverse engineering - it's all pIntf->vTable->func(pIntf, ...). You can explain it on a whiteboard in a couple of minutes.

The way MSVC does it may need reverse engineering (it may be patented btw). I could explain how Delphi implements COM interfaces, but any specific implementation is actually more complicated than the ABI, because they're trying to add implementation ergonomics on top of the basic calling convention.

[holy_city]

Is the ABI documented anywhere? Every time I google around for it, I just get information like "COM is ABI stable and language agnostic" but not what the ABI is. I've successfully implemented implementations of single COM interfaces and get the basics, my trouble was in implementing many interfaces for the same implementation and running into copious segfaults when testing the Rust implementation through a reference app written in C++.

[ChrisSD]

> The structs are defined in terms of C with Winapi (stdcall) calling convention.

This isn't quite right. The calling convention is a Windows C++ variant of stdcall. See this issue: https://github.com/rust-lang/rfcs/issues/1342

However, 99% of the time the difference won't be an issue.

[pjmlp]

COM layout is followed upon most mainstream Windows compiled languages, namely major C++ compilers, .NET, Delphi, Eiffel, Ada, so it is not MSVC++ keeping ABI compatibility under the hood on their own.

[holy_city]

My issue isn't ABI stability but the ABI itself w.r.t vtable layout. Best I can tell it should be similar to Itanium's spec? [1]. It's been months since I did this, but iirc my problems stemmed from having multiple interfaces on top of the same implementation and the ordering/layout of those interfaces, though the IUnknown interface which is supposed to handle that.

[1] https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vtable