81 lines
2.7 KiB
C++
81 lines
2.7 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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template<bool B, class T = void> // Default template version.
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struct enable_if {
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}; // This struct doesn't define "type" and the substitution will fail if you try to access it.
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template<class T> // A specialisation used if the expression is true.
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struct enable_if<true, T> {
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typedef T type;
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}; // This struct do have a "type" and won't fail on access.
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template<class T>
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typename enable_if<hasSerialize<T>::value, std::string>::type serialize(const T &obj) {
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return obj.serialize();
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}
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template<class T>
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typename enable_if<!hasSerialize<T>::value, std::string>::type serialize(const T &obj) {
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return to_string(obj);
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}
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int main() {
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// Usage:
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// enable_if<true, int>::type t1; // Compiler happy. t's type is int.
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// enable_if<hasSerialize<B>::value, int>::type t2; // Compiler happy. t's type is int.
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//
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// enable_if<false, int>::type t3; // Compiler unhappy. no type named 'type' in 'enable_if<false, int>';
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// enable_if<hasSerialize<A>::value, int>::type t4; // no type named 'type' in 'enable_if<false, int>';
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A a;
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B b;
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C c;
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// The following lines work like a charm!
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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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} |