static member function with C language binding?

Question

The following C++ code compiles with Visual C++ and g++:

struct S
{
    static void foo();
};

extern "C"
void S::foo() {}

struct T
{
    static void foo();
};

extern "C"
void T::foo() {}

auto main() -> int
{
    S().foo();
    T().foo();
}

• Is it valid?

• If it's valid, since the implementation may be in a separate translation unit, does that imply that a static member function always has the same calling convention as a C function (and if not, how does it not imply that)?

Answer 1

C++11 7.5/4 "Linkage specifications"

A C language linkage is ignored in determining the language linkage of the names of class members and the function type of class member functions.

So your example is valid in the sense that it's not malformed or an error, but the extern "C" should have no effect on S::foo() or T::foo().

Answer 2

A static member function has the same calling convention as a C function. But, name mangling applies. So, even if you get away with declaring your static member as extern "C", the linker would probably not find it when you try to link it against the C code that calls that function.

What you can easily do is declare a wrapper/stub that calls the static member from a plain function. Also, you can assign the static member function's address to a plain function pointer.

Answer 3

No it is ignored, the problem is name mangling (function naming for linkage phase). So the trick is to define a C function and use your C++ static method as a stub to call it, like this:

struct S
{
    static void foo();
};

extern "C" void S_foo_impl();
void S::foo() { S_foo_impl(); }

auto main() -> int
{
    S::foo();
}

Of course, S_foo_impl should be defined in a external C module.