[Mono-dev] Summer of Code / C++ Interop

[Andreas Färber]

Hi,

Am 08.04.2010 um 11:25 schrieb Alex Corrado:

> I am proposing to expand Mono's C++ interop support to enable the  
> creation of managed wrappers directly around native C++ objects.
[...]
> The first place I read about calling C++ functions directly from  
> managed code was on Mono's "Interop with Native Libraries" page. It  
> suggested setting the EntryPoint of the DllImport attribute to the C+ 
> + mangled function name to call that function directly through P/ 
> Invoke. However, it wasn't until I read this blog post by Jim  
> Springfield that I realized that, not only could this be a viable  
> technique, but that by messing with virtual tables, native C++  
> classes could effectively be subclassed by managed code. This  
> technique could allow for seamless managed wrappers around native C+ 
> + classes.
>
> Jim Springfield's example is tied completely to Microsoft's Visual C+ 
> + compiler, and this illustrates the largest problem with this  
> approach: that C++ ABIs are different among different compilers and  
> even between different versions of the same compiler. To help  
> ameliorate this issue, I have taken the basic principles in  
> Springfield's design and abstracted out any ABI-specific components  
> into an abstract class. A different subclass of this CppAbi class  
> can then be implemented to support each compiler's different name  
> mangling schemes and other idiosyncrasies. At runtime, the correct  
> CppAbi instance can be chosen when loading the C++ library depending  
> on platform or other conditions. Reflection.Emit is then used to  
> generate the P/Invoke code and implement trampolines to facilitate  
> virtual method calls. Eventually I hope to support seamless  
> exception handling and other features supported by C++/CLI on Windows.
>
> I realize this sounds very ambitious, but I've already implemented a  
> proof-of-concept based on a simple C++ class, similar to the one Jim  
> Springfield uses in his example. It is hosted on Google code at http://code.google.com/p/cppinterop/ 
> . Please note that this really is just a proof-of-concept-- I've  
> only implemented the Itanium C++ ABI, and only in part. If you are  
> using a recent version of GCC to compile C++, you should be able to  
> compile the example and call it directly from managed code. I've  
> only tested this on an Intel Mac running OS X 10.4.11.

I've recently investigated ways to p/invoke C++ code myself and  
considered going the name-mangling way, so this sounds interesting!  
Can't comment on whether it's suitable for GSoC though.

CSimpleClass.cs looks as if it was written manually. I see a problem  
with changing C++ code there: To allow managed code to instantiate  
such a class, your private struct needs to match exactly the size of  
the native code. If however someone adds a private field in C++ but  
does not update the interop code, it will fail. Would it be possible  
to leave the memory allocation to C++ (the ABI document mentions "nw"  
in the name-mangling section) and in C# just map the methods we  
actually want to call?

For your proposed project, would you be focussing on the p/invoke ABI  
infrastructure only? Any plans for SWIG-like autogeneration of the  
(C#) proxy interfaces from C++ headers? And what about C++ interop  
inside Mono's class libraries?

Regards,
Andreas