Call foreign member functions using Panama

[Maurizio Cimadamore]

Hi,
as others have pointed out in this thread, while supporting C++ from 
jextract/Panama is not impossible, there are several obstacles to get 
there. The biggest one is that the C++ platform compiler plays a very 
important role in how things get compiled (e.g. which mangled name an 
overloaded member function will get, among other things). Often, 
high-level parsing libraries such as libclang do not offer enough 
information for jextract to be able to reconstruct the full picture. 
Even ignoring low-level details such as vtables and inline functions, 
when we have tried to create a small wrapper around jextract to "lower" 
C++ code into C we ran into several problems esp. around templates, as 
the clang API was not exposing information about template instantiation 
correctly. When researching the problem, we realized that we were not 
alone, and that e.g. the Rust binding generator tool also suffers from 
similar issues/limitations:

https://rust-lang.github.io/rust-bindgen/cpp.html

Creating a foreign linker implementation based on LLVM bitcode would of 
course be a much more stable option (and the Linker interface should be 
generic enough to allow for such implementations). But going down that 
path would of course introduce additional constraints: users would need 
their C++ libraries to be compiled into LLVM bitcode. While that might 
be a possible move for 3rd party libraries (although I'd say still 
rather uncommon), this present problems when considering system 
libraries (here, GCC/MSVC still rule on Linux/Windows respectively). So, 
while the LLVM bitcode seems like an interesting target, I believe 
something "messier" is going to be needed in the short term (probably a 
combination of Java fooreign code plus some common C++ library, such as 
what JavaCPP does, or as described here [1]).

Maurizio

[1] - http://cr.openjdk.java.net/~jrose/panama/cppapi.cpp.txt

On 26/11/2022 06:57, Red IO wrote:
> I was building a small framework to connect rust and java (jdk 11) for 
> a project. I build a java class with 1 field a long. Then I added non 
> static native functions to the class representing the member functions 
> of the corresponding rust struct. I mangled myself through the jni 
> nightmare and stored a reference to the rust struct in the long. This 
> construct simulates first class foreign member function support pretty 
> well. After building this I thought about Panama. As far as I know 
> Panama currently only supports C (no concept of member functions). 
> Wouldn't it be a good idea to support languages like c++, rust and 
> their member functions? I mean with Panama we can already create 
> foreign structs. Would it be so far fetched to integrate constructors 
> and member functions into Panama and jextract?
>
> Great regards
> RedIODev