Get function pointer from std::function when using std::bind

Question

I\'m trying to use std::function in conjunction with std::bind, but I\'m having some problems.

This works:

#include 

        

Answer 1

This can be achieved using a little template meta-programming. I recently had use for this while writing a generic C++ wrapper around OpenGL GLUT (which depends on callback function pointers). The approach:

1. Instantiate an instance of a singleton template type.
2. Store your std::function as a member of to the singleton instance
3. Invoke your std::function through a static member function (static member functions and free functions have the same type, so the "invoke" function can be used as a free function pointer)

Tested under C++11 on GCC 4.8.

#include 
#include 
#include 
#include 
#include 
#include 
#include 

template 
struct fun_ptr_helper
{
public:
    typedef std::function<_Res(_ArgTypes...)> function_type;

    static void bind(function_type&& f)
    { instance().fn_.swap(f); }

    static void bind(const function_type& f)
    { instance().fn_=f; }

    static _Res invoke(_ArgTypes... args)
    { return instance().fn_(args...); }

    typedef decltype(&fun_ptr_helper::invoke) pointer_type;
    static pointer_type ptr()
    { return &invoke; }

private:
    static fun_ptr_helper& instance()
    {
        static fun_ptr_helper inst_;
        return inst_;
    }

    fun_ptr_helper() {}

    function_type fn_;
};

template 
typename fun_ptr_helper<_UniqueId, _Res, _ArgTypes...>::pointer_type
get_fn_ptr(const std::function<_Res(_ArgTypes...)>& f)
{
    fun_ptr_helper<_UniqueId, _Res, _ArgTypes...>::bind(f);
    return fun_ptr_helper<_UniqueId, _Res, _ArgTypes...>::ptr();
}

template
std::function::value, T>::type>
make_function(T *t)
{
    return {t};
}

int main()
{
    std::cout << (void*)get_fn_ptr<0>(make_function(::sin))<