获得函数返回值类型、参数tuple、成员函数指针中的对象类型

//function_traits.h，获得函数返回值类型、参数tuple、成员函数指针中的对象类型
//参考https://github.com/qicosmos/cosmos/blob/master/function_traits.hpp，进行了精简和补充
#pragma once
#include <functional>
#include <tuple>

//类模板原型
template<typename t>
struct function_traits;

//普通函数
template<typename ret, typename... args>
struct function_traits<ret(args...)> {
	typedef ret return_type;
	typedef std::tuple<std::remove_const_t<std::remove_reference_t<args>>...> bare_tuple_type;
};

//函数指针
template<typename ret, typename... args>
struct function_traits<ret(*)(args...)> : function_traits<ret(args...)>{};

//std::function.
template <typename ret, typename... args>
struct function_traits<std::function<ret(args...)>> : function_traits<ret(args...)>{};

//成员函数，以及成员函数指针中的对象类型
#define function_traits(...)\
template <typename ret, typename obj, typename... args>\
struct function_traits<ret(obj::*)(args...) __va_args__> : function_traits<ret(args...)>\
{ typedef obj object_type; };

function_traits()
function_traits(const)
function_traits(volatile)
function_traits(const volatile)

//函数对象
template<typename callable>
struct function_traits : function_traits<decltype(&callable::operator())>{};

template <typename fun>
typename function_traits<fun>::stl_function_type to_function(const fun& lambda) {
	return static_cast<typename function_traits<fun>::stl_function_type>(lambda);
}

template <typename fun>
typename function_traits<fun>::stl_function_type to_function(fun&& lambda) {
	return static_cast<typename function_traits<fun>::stl_function_type>(std::forward<fun>(lambda));
}

template <typename fun>
typename function_traits<fun>::pointer to_function_pointer(const fun& lambda) {
	return static_cast<typename function_traits<fun>::pointer>(lambda);
}