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