support bazel complie this project and format code.

learn_class/modern_cpp_30/SFINAE/sfinae paper/combiningAndGenius.cpp

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+//
+// Created by light on 20-1-6.
+//
+
+#include <iostream>
+#include "structData.h"
+
+
+template<class T>
+struct hasSerialize {
+    // 编译时比较
+    typedef char yes[1];
+    typedef char no[2];
+    // 允许我们检查序列化确实是一种方法
+    // 第二个参数必须是第一个参数的类型
+    // 例如： reallyHas<int,10> 替换为 reallyHas<int,int 10> 并起作用
+    // 注意：它仅适用于整数常量和指针(因此函数指针可以使用)
+    // 例如：reallyHas<std::string (C::*)(), &C::serialize> 替换为
+    // reallyHas<std::string (C::*)(), std::string (C::*)() &C::serialize> 并起作用
+    template<typename U, U u>
+    struct reallyHas;
+
+    // std::string (C::*)() 是函数指针声明
+    template<typename C>
+    static yes &test(reallyHas<std::string (C::*)(), &C::serialize> * /*unused*/) {}
+
+    //  ()()const 函数指针 -> std::string serialize() const
+    template<typename C>
+    static yes &test(reallyHas<std::string (C::*)() const, &C::serialize> * /*unused*/) {}
+
+    // The famous C++ sink-hole.
+    // Note that sink-hole must be templated too as we are testing test<T>(0).
+    // If the method serialize isn't available, we will end up in this method.
+    template<typename>
+    static no &test(...) { /* dark matter */ }
+
+    //用作测试的返回值的常数。
+    //由于编译时评估的大小，因此实际上在这里完成了测试。
+    static const bool value = sizeof(test<T>(0)) == sizeof(yes);
+};
+
+// Using the previous A struct and hasSerialize helper.
+struct D : A {
+    std::string serialize() const {
+        return "I am a D!";
+    }
+};
+
+template<class T>
+bool testHasSerialize(const T & /*t*/) { return hasSerialize<T>::value; }
+
+// 不能够判断仿函数里面的serialize
+struct E {
+    struct Functor {
+        std::string operator()() {
+            return "I am a E!";
+        }
+    };
+
+    Functor serialize;
+};
+
+template<class T>
+std::string serialize(const T &obj) {
+    // 不加constexpr 报错：error: no member named 'serialize' in 'A'.
+    if constexpr (hasSerialize<T>::value)
+        return obj.serialize();
+    else
+        return to_string(obj);
+}
+
+int main() {
+    // 检测结构体是否有serialize方法
+    // Using the struct A, B, C defined in the previous hasSerialize example.
+    std::cout << hasSerialize<A>::value << std::endl;
+    std::cout << hasSerialize<B>::value << std::endl;
+    std::cout << hasSerialize<C>::value << std::endl;
+
+
+    D d;
+    A &a = d; // Here we lost the type of d at compile time.
+    std::cout << testHasSerialize(d) << std::endl; // Output 1.
+    std::cout << testHasSerialize(a) << std::endl; // Output 0.
+
+    E e;
+    std::cout << e.serialize() << std::endl; // Succefully call the functor.
+    std::cout << testHasSerialize(e) << std::endl; // Output 0.
+
+
+    A a_;
+    B b_;
+    C c_;
+    std::cout << serialize(a_) << std::endl;
+    std::cout << serialize(b_) << std::endl;
+    std::cout << serialize(c_) << std::endl;
+}