update

modern_C++_30/compilercompute/WhileLoop.cpp

@@ -0,0 +1,77 @@
+//
+// Created by light on 20-1-5.
+//
+
+#include <iostream>
+
+using namespace std;
+
+// 使用template实现while循环
+
+template<bool condition,
+        typename Body>
+struct WhileLoop;
+
+template<typename Body>
+struct WhileLoop<true, Body> {
+    typedef typename WhileLoop<
+            Body::cond_value,
+            typename Body::next_type>::type
+            type;
+};
+
+template<typename Body>
+struct WhileLoop<false, Body> {
+    typedef
+    typename Body::res_type type;
+};
+
+template<typename Body>
+struct While {
+    typedef typename WhileLoop<Body::cond_value, Body>::type type;
+};
+template<typename Body>
+using While_t = WhileLoop<Body::cond_value, Body>;
+
+namespace my {
+    template<class T, T v>
+    struct integral_constant {
+        static const T value = v;
+        typedef T value_type;
+        typedef integral_constant<T, v> type;
+    };
+}
+template<int result, int n>
+struct SumLoop {
+
+    // 循环的条件
+    static const bool cond_value =
+            n != 0;
+
+    // 循环后的结果
+    static const int res_value =
+            result;
+
+    // 循环时的状态
+    typedef my::integral_constant<
+            int, res_value>
+            res_type;
+
+    // 循环执行一次时的状态
+    typedef SumLoop<result + n, n - 1>
+            next_type;
+};
+
+template<int n>
+struct Sum {
+    typedef SumLoop<0, n> type;
+};
+
+template<int n>
+using Sum_t = SumLoop<0, n>;
+
+int main() {
+    cout << While<Sum<6>::type>::type::value << endl;
+    cout << While_t<Sum_t<6>>::type::value << endl;
+    return 0;
+}