How to determine number of function arguments dynamically - c++11

Question

I have the following code:
#include <iostream>
#include <functional>
class test
{
public:
typedef std::function<bool(int)> Handler;
void handler(Handler h){h(5);}
};
class test2
{
public:
template< typename Ret2, typename Ret, typename Class, typename Param>
inline Ret2 MemFn(Ret (Class::*f)(Param), int arg_num)
{
if (arg_num == 1)
return std::bind(f, this, std::placeholders::_1);
}
bool f(int x){ std::cout << x << std::endl; return true;}
};
int main()
{
test t;
test2 t2;
t.handler(t2.MemFn<test::Handler>(&test2::f, 1));
return 0;
}
It works as expected.
I would like to be able to call this:
t.handler(t2.MemFn<test::Handler>(&test2::f));
instead of
t.handler(t2.MemFn<test::Handler>(&test2::f, 1));
Basically I need MemFn to determine in runtime what Handler expects as the number of arguments.
Is that even possible?

Answer 1

You may create some type_traits to have your info, something like:
template <typename T> struct function_trait;
template <typename Ret, typename ... Args>
struct function_trait<std::function<Ret(Args...)>>
{
static constexpr std::size_t args_count = sizeof...(Args);
};
And so your method may look like:
template<typename Ret2, typename Ret, typename Class, typename Param>
inline Ret2 MemFn(Ret (Class::*f)(Param))
{
if (function_trait<Ret2>::args_count == 1)
return std::bind(f, this, std::placeholders::_1);
throw std::runtime_error("invalid number of arguments");
}