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std::chrono:: operator==,<=> (std::chrono::year_month)

From cppreference.net
C++
Date and time library
std::chrono::year_month
定义于头文件 <chrono>
constexpr bool operator == ( const std:: chrono :: year_month & x,
const std:: chrono :: year_month & y ) noexcept ;
(1) (C++20 起)
constexpr std:: strong_ordering operator <=> ( const std:: chrono :: year_month & x,
const std:: chrono :: year_month & y ) noexcept ;
(2) (C++20 起)

比较两个 year_month 值 x 和 y 。

< 、 <= 、 > 、 >= 和 != 运算符分别由 operator <=> 和 operator == 合成。

返回值

1) x. year ( ) == y. year ( ) && x. month ( ) == y. month ( )
2) x. year ( ) <=> y. year ( ) ! = 0 ? x. year ( ) <=> y. year ( ) : x. month ( ) <=> y. month ( )

示例

运行此代码 
#include <chrono>
#include <iostream>
int main()
{
    using namespace std::chrono;
    constexpr year_month ym1{year(2021), month(7)};
    constexpr year_month ym2{year(2021)/7};
    static_assert(ym1 == ym2);
    std::cout << ym1 << '\n';
    static_assert((2020y/1 < 2020y/2) && (2020y/2 == 2020y/2) && (2020y/3 <= 2021y/3) &&
                  (2023y/1 > 2020y/2) && (3020y/2 != 2020y/2) && (2020y/3 >= 2020y/3));
}

输出： 

2021/Jul