function_traits.hpp

/*
* Usage:
* void test(string s, int n) { }
* cout << function_traits<decltype(test)>::arity << endl; //< Prints 2.
* function_traits<decltype(test)>::arg<0>::type s; //< Declares a string.
* function_traits<decltype(test)>::arg<1>::type n; //< Declares an int.
*/

namespace utility {
	template<typename FunctionType>
	struct function_traits : function_traits<std::function<FunctionType>> {};

	template<typename Ret, typename ...Args>
	struct function_traits<std::function<Ret(Args...)>> { //< Exploit std::function to unpack arguments.
		using return_type = Ret; //< Return type of the function.
		using args_tuple  = std::tuple<Args...>; //< A tuple of the function's arguments.
		static const size_t arity = sizeof...(Args); //< Number of arguments.

		template <size_t N>
		struct arg {
			// Exploit tuple_element to get the type of an argument.
			using type = typename std::tuple_element<N, std::tuple<Args...>>::type; //< Type of the nth argument.
		};
	};
}