on languages that can link to c++

Question

As I was reflecting on this subject, it seemed to me that if a language is implemented in C++, it might well have a mechanism for linking to C++. As I recall, Java is this way through JNI, though I don't really remember if it goes through C++ or plain C.

However, it seems that in general languages don't link to C++, and are accessible to C++ only through C for various reasons.

So, out of curiosity,

What languages are there that can link to C++ and how and to what extent?

(No credit for the aforementioned C bridge unless it is done in an elegant or interesting way, like Boost.Python)

Answer 1

Note: the ABI includes name-mangling as well as the calling convention, environment requirements, etc. The EABI (not covered here) is platform-specific but is usually standardized for each platform (though this is not guaranteed) and does not suffer from the problems of a variadic EABI (because it doesn't change much). Consult ABI on Wikipedia for a bit more on the subject.

Linking to C++ is unlikely because the ABI is undefined. Thus, the names of symbols can change at the will of the compiler vendor. GCC does it differently from MS's compiler, for example. Even with the same compiler ABI's may be different between versions (as is the case with some GCC versions).

However, linking to C++ is not impossible. It's common for C++ programs to link to other C++ programs because both are compiled by the same compiler (or at least compilers which use the same ABI). If the ABI is known, non-C++ applications can link to C++ ones.

Linking to C is easy. This is because C's ABI is more stable than C++'s ABI for most platforms. This is due to C's simplicity compared to C++. Because C++ can export C symbols, bypassing the problem with C++'s ABI, you sometimes see C++ libraries export C symbols which allow interaction with the underlying C++ objects without the problems of name mangling, link compatability, and future-proofing. However, doing this prevents you from easily using the C++ library via the C interface with the C++ interface.

But you can use the C++ interface by using the C exports. The library can provide some classes in header files which wrap the C exports for you. This is more work on the library writers (and can cause some problems because DRY usually isn't practiced with this technique) but provides much benefit for library users (which can choose between both interfaces without worrying about linking issues!). For other languages the library writers can provide interfaces like the C++ header files which deal with the C exports themselves. (You then have the advantage of using native classes and such.)

Answer 2

There is no language that can do this well because the C++ standard does not specify a name mangling scheme. Because of this, different compilers are free to mangle names differently, and there's no consistent way to link to C++ binaries.

The reason so many things can link to C is because C has simple and consistent linkage. A function foo is called foo in the object file. Likewise, things can link to Fortran reasonably well because foo is one of foo, FOO, foo_, or foo__.

This is why there are so many wrapper generators for C++ objects (SWIG, Boost.Python, SIP, etc.). They define an interface as a set of C calls precisely because that makes things easy to link against.

Something else to keep in mind is whether you really want to link directly to C++ libraries. Many wrapper generators provide a lot of policy options when you generate wrappers. Remember, C calls boil down to just functions, but C++ calls typically have a lot of OO semantics you'll need to navigate around. You need to specify how your C++ objects should be garbage collected in the host language, how things get copied, where vptrs are, how objects are laid out, where objects get allocated, and all the subtle differences between the host language's object model and that of C++.

I know it sounds ideal to just link to a C++ library and be done with it, but it's not that simple, and you're going to want more than just direct linkage in the end anyway.

Answer 3

Looked at SWIG lately?