C++(20)：三路比较运算符

C++20增加了三路比较运算符<=>（戏称航天飞机运算符），用于对类的比较运算符进行统一的设计。

有两种使用方式：

1. 默认比较

对于某些类，如果按照其成员逐一比较即可决定比较运算符的值，那么可以使用默认的三路运算符，编译器为为类生成==,!=,<,>,<=,>=

#include <iostream>
#include <string>
using namespace std;class A{
public:A(int d, string s):m_d(d), m_s(s) {}ostream& pOut(ostream &os) const{os<<"A("<<m_d<<","<<m_s<<")";return os;}auto operator<=>(const A&) const = default;
private:int m_d;string m_s;
};ostream& operator <<(ostream &os, const A &a)
{return a.pOut(os);
}void doCompare(const A& d1, const A& d2)
{if(d1 > d2){cout<<d1<<" is bigger than "<<d2<<endl;}else if(d1 == d2){cout<<d1<<" is equal to "<<d2<<endl;}else if(d1 < d2){cout<<d1<<" is smaller than "<<d2<<endl;}
}int main()
{A a1(1, "hi"), a2(1, "hello"), a3(2, "a");doCompare(a1, a2);doCompare(a1, a1);doCompare(a1, a3);return 0;
}

运行程序输出：
A(1,hi) is bigger than A(1,hello)
A(1,hi) is equal to A(1,hi)
A(1,hi) is smaller than A(2,a)

默认的比较运算符将按照成员的顺序先比较m_d然后再比较m_s

如果将m_d与m_s定义的顺序反过来，那么会影响到比较的结果：

#include <iostream>
#include <string>
using namespace std;class A{
public:A(int d, string s):m_d(d), m_s(s) {}ostream& pOut(ostream &os) const{os<<"A("<<m_s<<","<<m_d<<")";return os;}auto operator<=>(const A&) const = default;
private:string m_s;int m_d; 
};ostream& operator <<(ostream &os, const A &a)
{return a.pOut(os);
}void doCompare(const A& d1, const A& d2)
{if(d1 > d2){cout<<d1<<" is bigger than "<<d2<<endl;}else if(d1 == d2){cout<<d1<<" is equal to "<<d2<<endl;}else if(d1 < d2){cout<<d1<<" is smaller than "<<d2<<endl;}
}int main()
{A a1(1, "hi"), a2(1, "hello"), a3(2, "a");doCompare(a1, a2);doCompare(a1, a1);doCompare(a1, a3);return 0;
}

运行程序输出：
A(hi,1) is bigger than A(hello,1)
A(hi,1) is equal to A(hi,1)
A(hi,1) is bigger than A(a,2)

2. 定制比较

当默认的比较规则不合适时，可以通过定义<=>为类生成统一的比较运算符。

定制比较时可以分为三个级别，对应的返回值类型分别为：

返回类型	是否有无法比较的值	说明
(强序)std::strong_ordering	无	所有元素都可比较，且有严格的顺序关系（可比较，等值唯一），比如数值
(弱序)std::weak_ordering	无	所有元素都可比较，但存在近似相等的情况（可比较，等值不唯一），比如大小写字符
(偏序)std::partial_ordering	是	元素存在不可比较的情况（非全可比较，等值也不唯一），比如void*指针

以下实现一个不区分大小写的字符串比较：

#include <iostream>
#include <string>
using namespace std;auto strToUpper = [](string str)  
{string s = str;transform(s.begin(), s.end(), s.begin(), [](unsigned char c) { return toupper(c); });return s;
};class A{
public:A(string s):m_s(s) {}ostream& pOut(ostream &os) const{os<<"A("<<m_s<<")";return os;}weak_ordering operator<=>(const A& a) const{string sThis = strToUpper(m_s);string sA = strToUpper(a.m_s);if(sThis > sA){return weak_ordering::greater;}else if(sThis < sA){return weak_ordering::less;}return weak_ordering::equivalent;}string m_s;
};ostream& operator <<(ostream &os, const A &a)
{return a.pOut(os);
}void doCompare(const A& d1, const A& d2)
{if(d1 > d2){cout<<d1<<" is bigger than "<<d2<<endl;}else if(d1 < d2){cout<<d1<<" is smaller than "<<d2<<endl;}else{cout<<d1<<" is equal to "<<d2<<endl;}
}int main()
{A a1("hi"), a2("hello"), a3("Hi"), a4("ho");doCompare(a1, a2);doCompare(a1, a3);doCompare(a1, a4);return 0;
}

运行程序输出：
A(hi) is bigger than A(hello)
A(hi) is equal to A(Hi)
A(hi) is smaller than A(ho)

可以看到通过实现了<=>编译器会由此实现其他运算符。