callable.cpp

#include <iostream>
#include <typeinfo>
#include <type_traits>
#include <functional>
#include <tuple>

using namespace std;

int TestCount = 0;

struct Test
{
    int t;
    Test() { t = ++TestCount; cout << t << "Test()" << endl; }
    ~Test() { cout << "~Test()" << endl; }
    Test(const Test &) { t = ++TestCount; cout << t << "Test(const Test &)" << endl; }
    Test(Test &&) { cout << t << "Test(Test &&)" << endl; }
    Test & operator=(const Test &) { cout << t << "operator=(const Test &)" << endl; }
    Test & operator=(Test &&) { cout << t << "operator=(Test &&)" << endl; }

};

template <typename T>
struct callable_type : callable_type<decltype(&T::operator())>
{};

template <typename ClassType, typename ReturnType, typename... Args>
struct callable_type<ReturnType(ClassType::*)(Args...) const>
{
    using return_type = ReturnType;
    using argument_type = std::tuple<Args...>;
    
    template<std::size_t I>
    using arguments = std::tuple_element<I, argument_type>;
};

template <class T>
void test(T t) 
{
    t();
}

int main()
{
    auto f = [](int i, float, bool){ return (double)i; };
    cout << boolalpha
    << is_same<callable_type<decltype(f)>::arguments<0>::type, int>::value << endl
    << is_same<callable_type<decltype(f)>::arguments<1>::type, float>::value << endl
    << is_same<callable_type<decltype(f)>::arguments<2>::type, bool>::value << endl
    ;
}