更多的宏，避免重复代码。

src/群友提交/第10题/494199.cpp

@@ -1,7 +1,11 @@
 #include <iostream>
 #include <utility>
 namespace {
-	struct _init {
+	struct _any {
+		//为了更好看的扩展ints的语法
+		explicit _any(auto);
+
+		//convert to any
 		template<class T>
 		operator T();
 	};
@@ -10,7 +14,7 @@ namespace {
 	constexpr bool _can_size(std::integer_sequence<size_t, ints...> int_seq)
 	{
 		return (requires{
-			T{(ints, _init{})...};
+			T{ _any(ints)... };
 		});
 	}
 
@@ -45,30 +49,39 @@ namespace {
 	}
 }
 
+//为了能递归使用LOOP
+//https://stackoverflow.com/a/12540675
+# define EMPTY(...)
+# define DEFER(...) __VA_ARGS__ EMPTY()
+# define OBSTRUCT(...) __VA_ARGS__ DEFER(EMPTY)()
+# define EXPAND(...) __VA_ARGS__
+
 #define LOOP0(body,sep)
-#define LOOP1(body,sep) body(0)
-#define LOOP2(body,sep) LOOP1(body,sep) sep() body(1)
-#define LOOP3(body,sep) LOOP2(body,sep) sep() body(2)
-#define LOOP4(body,sep) LOOP3(body,sep) sep() body(3)
+#define LOOP1(body,sep) body(1)
+#define LOOP2(body,sep) body(2) sep() LOOP1(body,sep) 
+#define LOOP3(body,sep) body(3) sep() LOOP2(body,sep) 
+#define LOOP4(body,sep) body(4) sep() LOOP3(body,sep) 
 
-//LOOP(2) EXPAND TO: body(0) sep() body(1)
-#define LOOP(n,body,sep) LOOP ## n(body,sep)
 
+//从n到1的重复循环
+//LOOP(n) EXPAND TO: body(n) sep() body(n-1) ... body(1)
+#define LOOP(n,body,sep) LOOP ## n(body,sep)
+#define LOOP_ID() LOOP
 
 template<class T>
 void for_each_member(const T& obj, auto&& f)
 {
-#define EMPTY()
+	//#define EMPTY()
 #define COMMA() ,
 
-//m0
+	//m0
 #define MAKE_M_VAR(n) m##n
 //f(m0)
 #define MAKE_F_CALL(n) f(m##n)
 //const auto& [m0, m1, m2, m3] = obj;
-#define MAKE_UNPACK(n) const auto& [ LOOP(n,MAKE_M_VAR,COMMA) ] = obj
+#define MAKE_UNPACK(n) const auto& [ DEFER(LOOP_ID)()(n,MAKE_M_VAR,COMMA) ] = obj
 //f(m0), f(m1), f(m2), f(m3);
-#define MAKE_CALL(n) LOOP(n, MAKE_F_CALL, COMMA);
+#define MAKE_CALL(n) DEFER(LOOP_ID)()(n, MAKE_F_CALL, COMMA);
 
 //MAKE_ALL_CHECK(4) EXPAND TO:
 //if constexpr (size<T>() == 4)
@@ -83,10 +96,25 @@ void for_each_member(const T& obj, auto&& f)
 		MAKE_CALL(n);\
 	}
 
-	MAKE_ALL_CHECK(4);
-	MAKE_ALL_CHECK(3);
-	MAKE_ALL_CHECK(2);
-	MAKE_ALL_CHECK(1);
+	//这里LOOP被递归调用了
+
+	/*
+	expand to:
+
+	if constexpr (size<T>() == 4) {
+		const auto& [m4, m3, m2, m1] = obj; f(m4), f(m3), f(m2), f(m1);;
+	} if constexpr (size<T>() == 3) {
+		const auto& [m3, m2, m1] = obj; f(m3), f(m2), f(m1);;
+	} if constexpr (size<T>() == 2) {
+		const auto& [m2, m1] = obj; f(m2), f(m1);;
+	} if constexpr (size<T>() == 1) {
+		const auto& [m1] = obj; f(m1);;
+	};
+	*/
+	EXPAND(LOOP(4, MAKE_ALL_CHECK, EMPTY));
+
+
+
 
 #undef MAKE_ALL_CHECK
 #undef MAKE_CALL
@@ -106,11 +134,10 @@ int main() {
 
 	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);
 }
-