Calling function template specialization using C calling conventions

Question

I have the following template

template void f(T t) { }

And I want to pass the address of a specific specialization of it

Answer 1

What does the C header look like? Somewhere, the C source must enumerate the callback types allowed. You should take that opportunity to have a series of macros that generate prototypes to individual stub functions, with a corresponding sequence of macros in the C++ source generating extern "C" stubs.

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As to the second question: Yes, that works, but the typedef is not inside a template. I attempted to put such a typedef inside a class, but it turns out that even class templates are not allowed inside the extern "C". So you can have a function template, but no parameters of dependent type.

Merely defining that function is easy:

extern "C" typedef void ftype(int);

template
static ftype f; // <- added "static" here

template< typename T >
void f(int q) {}

Aha, variadic functions!

extern "C" typedef void ftype( int, ... );

template
static ftype f;

template< typename T >
static void f( int z, ... ) {
    va_list va;
    va_start( va, z );
    T v = va_arg( va, T );
    va_end( va );

    std::cout << v;
}

You don't really need type deduction since it's just a callback, so you can pass this & f to the C code, all callbacks having the same type, and it can make the type determination at runtime and pass whatever it wants through the varargs.