[geofft]

Rust can call directly into a C++ library as long as it uses the C ABI: there aren't bindings to (a compiler-specific implementation of) the C++ type system. Go is just the same. This isn't really a language restriction, more a result of how rich the C type system is and how compiler-specific it is. The only languages that have direct C++ interop are those that directly use a C++ compiler (like Objective-C++).

It appears that GoQt has this giant pile of wrappers around C++ types, casting pointers back and forth:

https://raw.githubusercontent.com/visualfc/goqt/master/qtdrv...

I would have expected this to require an `extern "C"` declaration. I'm sort of surprised it doesn't, but maybe the common C++ ABIs match the C ones if only C types are passed to and from the function?

(Also, is this file auto-generated?)

[shmerl]

I wonder though why C++ couldn't come up with a standard for names decoration? It would solve tons of such issues.

[geofft]

Name decoration isn't the hard part; it's types that are hard. Remember that a simple string is a std::basic_string<char, std::char_traits<char>, std::allocator<char>>; passing a string from one place to another requires either sharing the latter two, or having a way to convey pointers to what the latter two do. And that's not even considering things like, oh, throwing an exception whose class uses multiple inheritance and virtual methods.

Take a look at the Itanium C++ ABI, the de facto standard on GNU/Linux etc. for a sense of the scale of the problem: https://mentorembedded.github.io/cxx-abi/abi.html

And that doesn't even standardize the layout of the actual types in std::* themselves, just the typesystem. That just gives you enough information to correctly access someone else's implementation of std::string; you still need to have that precise implementation around at compile time. There is, however, a proposal to standardize the layout of std: https://isocpp.org/files/papers/n4028.pdf

To be fair, you have the exact same problem when trying to call into e.g. Rust from e.g. C. Rust's typesystem is complex and its libstd is involved and not stabilized, so the easiest way is, again, to route through C types instead of trying to access Rust's String type in C.

[pcwalton]

It'd be an improvement if everyone used the Itanium ABI, though.

[shmerl]

> here is, however, a proposal to standardize the layout of std

That's interesting, thanks.

[pjmlp]

Compiler and OS vendors. The same behind undefined behavior.

Although people like to bash C++, there isn't any programming language with an ABI as part of the language standard, not even C.

The C ABI that so many people adhere to, only exists in OS written either in C or C++ (via extern "C").

By the historical accident that all commercial major OSes are mostly written in C, developers that aren't language lawyers tend to think C ABI is somehow defined in some standard.

In OSes that weren't written C, with C compilers available, like OS/400, VMS, Lillith, Lisp Machines, Oberon, the ABI being used isn't the C one.

[geofft]

Yes, the C ABI / calling convention is OS- and platform-specific. (Which does lead to practical problems on modern systems when you assume the ABI behaves similarly on different platforms; a good example is https://github.com/rust-lang/rust/pull/22862 , caused by 64-bit iOS treating varargs differently from OS X on x86-64 or even Linux on aarch64.)

However, within a given platform, the ABI tends to be small, clear, and well-documented. It's not documented in the C language standard, but there is documentation for it, from whoever defines the platform.

I thought I had mentioned this in another comment, but to be clear, I'm not trying to "bash C++"; plenty of other type systems and standard libraries have the same issue (including Go, Rust, Python, etc.). I'm just relaying the fact that, on the platforms where Go and Rust support FFI, they do so by implementing the C ABI, which is a stable ABI defined in the platform documentation, and the C++ ABI tends not to be well-defined or stable and is also much more complex to implement. This isn't a criticism of C++, just a fact. (And there are good things about this; for instance, C++'s type system is so much more useful than C's.)

[pjmlp]

Fair enough, my reaction was more to those that bash C++ without recognition that the issue is not C++ specific.

[emmelaich]

The answer is in the wikipedia article you linked to earlier...

    [it] would not suffice to guarantee C++ compiler
    interoperability and it might even create a false
    impression that interoperability is possible and
    safe when it isn't.

[geofft]

Oh, that's a very good point: if you have two different implementations of std::string, you want them to name-mangle differently so that you don't accidentally pass e.g. a GNU std::string to a function expecting an LLVM one.

[shmerl]

It doesn't really answer the question. It's not only about compilers interoperability, but about improving situation with bindings from other languages. And making name mangling standard would surely improve it (at least somewhat).