Get argument type of template callable object

Question

Consider the following function:

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


        

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 &);
};

the type of argument is an ambiguity here.