mq日提交第十题 (#314)

* mq日提交第十题

* 添加了头文件

src/群友提交/第10题/mq日.cpp

@@ -0,0 +1,103 @@
+#include <iostream>
+#include <type_traits>
+using namespace std;
+
+template <unsigned I>
+struct tag : tag<I - 1> {};
+
+template <>
+struct tag<0> {};
+
+struct init{
+	template <typename T>
+	operator T();
+};
+
+template <typename T>
+constexpr auto size_(tag<4>)
+	-> decltype(T{init{}, init{}, init{}, init{}}, 0u)
+{
+	return 4u;
+}
+
+template <typename T>
+constexpr auto size_(tag<3>)
+	-> decltype(T{init{}, init{}, init{}}, 0u)
+{
+	return 3u;
+}
+
+template <typename T>
+constexpr auto size_(tag<2>)
+	-> decltype(T{init{}, init{}}, 0u)
+{
+	return 2u;
+}
+
+template <typename T>
+constexpr auto size_(tag<1>)
+	-> decltype(T{init{}}, 0u)
+{
+	return 1u;
+}
+
+template <typename T>
+constexpr auto size_(tag<0>)
+	-> decltype(T{}, 0u)
+{
+	return 0u;
+}
+
+template <class T>
+constexpr int size()
+{
+	static_assert(std::is_aggregate_v<T>);
+	return size_<T>(tag<4>{});
+}
+
+template <typename T, typename F>
+void for_each_member(T const &v, F &&f)
+{
+	static_assert(std::is_aggregate_v<T>); 
+
+	if constexpr (size<T>() == 4u)
+	{ 
+		const auto &[m0, m1, m2, m3] = v;
+		f(m0);f(m1);f(m2);f(m3);
+	}
+	else if constexpr (size<T>() == 3u)
+	{
+		const auto &[m0, m1, m2] = v;
+		f(m0);f(m1);f(m2);
+	}
+	else if constexpr (size<T>() == 2u)
+	{
+		const auto &[m0, m1] = v;
+		f(m0);f(m1);
+	}
+	else if constexpr (size<T>() == 1u)
+	{
+		const auto &[m0] = v;
+		f(m0);
+	}
+}
+int main()
+{
+	struct X
+	{
+		std::string s{" "};
+	} x;
+	struct Y
+	{
+		double a{}, b{}, c{}, d{};
+	} y;
+	std::cout << size<X>() << '\n';
+	std::cout << size<Y>() << '\n';
+
+	auto print = [](const auto &member)
+	{
+		std::cout << member << ' ';
+	};
+	for_each_member(x, print);
+	for_each_member(y, print);
+}