c++ - Get argument type of template callable object

Question

Consider the following function:

template<class F>
void register_handler( F& f ) // any callable object
{
   // find out T - the argument type of f
}

Here f is some callable object, accepting one argument. It may be a function pointer, an std::function or a result of std::bind.

The problem is, how to determine the argument type of f and do some actions based on that type?

An easy workaround would be to add the type to template explicitly, like

template<class T, class F> // T is the argument type of F
void register_handler( F& f )

but this seems an overkill because type F should already contain the necessary information about type T.

Answer 1

Assuming F is any callable type, you cannot get its argument type. Consider this:

struct callable
{
    void operator() (int);
    void operator() (float *);
    void operator() (std::string const &);
    void operator() (std::list<int> &);
};

the type of argument is an ambiguity here.