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

I don't know the reason for the restriction, but can't you use a wrapper extern "C" function that calls the specific template instantiation you care about?

Answer 2

...whether there is an easy work around to achieve my initial goal?

you can approach it from a few angles, depending on how you want to configure and declare them.

four approaches follow, one namespace demonstrates each. 'Type' is probably the simplest for your usage.

#include <stdio.h> // favored to reduce exports in later demonstration

#define FUNC_NAME __PRETTY_FUNCTION__
// or __func__ or __FUNCTION__ or...

extern "C" {

    /* C prototype */
    typedef void ftype(int a);

    /* the c function all our functions are piped into */
    void call(ftype* a);

    /* helper which serves as the implementation for our functions */
    void print(int a, const char* const func);

    /* C definitions (used in namespace examples below) */
    static void static_float(int a) {
        print(a, FUNC_NAME);
    }

    static void static_double(int a) {
        print(a, FUNC_NAME);
    }

    void extern_float(int a);
    void extern_float(int a) {
        print(a, FUNC_NAME);
    }

    void extern_double(int a);
    void extern_double(int a) {
        print(a, FUNC_NAME);
    }

    static void static_function_float(int a) {
        print(a, FUNC_NAME);
    }

    static void static_function_double(int a) {
        print(a, FUNC_NAME);
    }

} /* << extern C */

namespace Extern {

    /**
     interface demonstrates C functions as template arguments
    */
    template<ftype Fn>
    struct t_func {
        static ftype* Function() {
            return Fn;
        }
    };

    template<typename T> struct bind;

    template<> struct bind<float> {
        typedef t_func<extern_float> F;
    };

    template<> struct bind<double> {
        typedef t_func<extern_double> F;
    };

    template<typename T>
    void Call(T a) {
        (void) a;
        call(bind<T>::F::Function());
    }

} /* << Extern */

namespace Static {

    /**
     interface demonstrates template types wrapping static C functions
     */
    template<typename T> struct bind;

    template<> struct bind<float> {
        static ftype* F() {
            return static_float;
        }
    };

    template<> struct bind<double> {
        static ftype* F() {
            return static_double;
        }

    };

    template<typename T>
    void Call(T a) {
        (void) a;
        call(bind<T>::F());
    }

} /* << Static */

namespace Function {

    /**
     interface demonstrates template functions wrapping static C functions
     */

    template<typename T> ftype* bind();

    template<> ftype* bind<float> () {
        return static_function_float;
    }

    template<> ftype* bind<double> () {
        return static_function_double;
    }

    template<typename T>
    void Call(T a) {
        (void) a;
        call(bind<T> ());
    }

} /* << Function */

namespace Type {

    /**
     interface demonstrates template types implementing static functions.
     although gcc4.2 and clang both compile it, i'm uncertain that this is conforming.
    */

    template<typename T> struct bind {
        static void F(int a);
    };

    template<> void bind<float>::F(int a) {
        print(a, FUNC_NAME);
    }

    template<> void bind<double>::F(int a) {
        print(a, FUNC_NAME);
    }

    template<typename T>
    void Call(T a) {
        (void) a;
        call(bind<T>::F);
    }

} /* << Type */

int main(int argc, const char * argv[]) {
    (void) argc;
    (void) argv;

    const float f(1.0f);
    const double d(5.0);

    Extern::Call(f);
    Extern::Call(d);

    Static::Call(f);
    Static::Call(d);

    Function::Call(f);
    Function::Call(d);

    Type::Call(f);
    Type::Call(d);

    return 0;
}

void call(ftype* a) {
    a(11);
}

void print(int a, const char* const func) {
    printf("%i: %s\n", a, func);
}

outputs:

11: void extern_float(int)
11: void extern_double(int)
11: void static_float(int)
11: void static_double(int)
11: void static_function_float(int)
11: void static_function_double(int)
11: static void Type::bind<T>::F(int) [with T = float]
11: static void Type::bind<T>::F(int) [with T = double]

producing:

nm unstripped:
    0000000100000daf s  stub helpers
    0000000100001048 D _NXArgc
    0000000100001050 D _NXArgv
    0000000100000bde T __ZN4Type4bindIdE1FEi
    0000000100000bc0 T __ZN4Type4bindIfE1FEi
    0000000100000d98 s __ZZ12extern_floatE19__PRETTY_FUNCTION__
    0000000100000c98 s __ZZ12static_floatE19__PRETTY_FUNCTION__
    0000000100000d80 s __ZZ13extern_doubleE19__PRETTY_FUNCTION__
    0000000100000cb0 s __ZZ13static_doubleE19__PRETTY_FUNCTION__
    0000000100000d60 s __ZZ21static_function_floatE19__PRETTY_FUNCTION__
    0000000100000d38 s __ZZ22static_function_doubleE19__PRETTY_FUNCTION__
    0000000100000cc8 s __ZZN4Type4bindIdE1FEiE19__PRETTY_FUNCTION__
    0000000100000d00 s __ZZN4Type4bindIfE1FEiE19__PRETTY_FUNCTION__
    0000000100001060 D ___progname
    0000000100000000 A __mh_execute_header
    0000000100001058 D _environ
                     U _exit
    0000000100000c00 T _extern_double
    0000000100000b20 T _extern_float
    0000000100000c20 T _main
                     U _printf
    0000000100000b60 t _static_double
    0000000100000b40 t _static_float
    0000000100000ba0 t _static_function_double
    0000000100000b80 t _static_function_float
                     U dyld_stub_binder
    0000000100000ae0 T start


nm stripped:
    0000000100000000 A __mh_execute_header
                     U _exit
                     U _printf
                     U dyld_stub_binder

sorry, i'm not poring over standards tonight -- hope that helps. good luck!

Answer 3

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.

---

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<typename T>
static ftype f; // <- added "static" here

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

Aha, variadic functions!

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

template<typename T>
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<int> 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.