# std::chrono::operator+, std::chrono::operator- (std::chrono::year)

< cpp‎ | chrono‎ | year

定义于头文件 `<chrono>` |  |
constexpr std::chrono::year operator+(const std::chrono::year& y, const std::chrono::years& ys) noexcept; | (1) | (C++20 起)
constexpr std::chrono::year operator+(const std::chrono::years& ys, const std::chrono::year& y) noexcept; | (2) | (C++20 起)
constexpr std::chrono::year operator-(const std::chrono::year& y, const std::chrono::years& ys) noexcept; | (3) | (C++20 起)
constexpr std::chrono::years operator-(const std::chrono::year& y1, const std::chrono::year& y2) noexcept; | (4) | (C++20 起)

1-2) 对 `y` 加 `ys.count()` 年。
3) 从 `y` 减去 `ys.count()` 年。
4) 返回 `y1` 与 `y2` 间按年计算的差。

### 返回值

1-2) std::chrono::year(int(y) + ys.count())
3) std::chrono::year(int(y) - ys.count())
4) std::chrono::years(int(y1) - int(y2))

### 注解

若 (1-3) 产生的年份值在范围 [-32767,32767] 外，则实际存储值未指定。

二个 `year` 值相减的结果是 std::chrono::years 类型时长。此时长单位表示格里高利年的平均长度，而结果与运算数所表示的具体年中的日数无关。例如 2018y - 2017y 的结果是 std::chrono::years(1) ，它表示 365.2425 日，而非 365 日。

### 示例

    #include <iostream>
    #include <chrono>

    int main()
    {

        std::chrono::year y {2020};

        y = y + std::chrono::years(12);
        if (y == std::chrono::year(2032)) {
            std::cout << "Years added correctly" << "\n";
        } else {
            std::cout << "Years not added correctly" << "\n";
        }

        y = y - std::chrono::years(33);
        if (y == std::chrono::year(1999)) {
            std::cout << "Years subtracted correctly" << "\n";
        } else {
            std::cout << "Years not subtracted correctly" << "\n";
        }

        std::chrono::years ys = std::chrono::year(2025) - std::chrono::year(2020);
        if (ys == std::chrono::years(5)) {
            std::cout << "Years subtracted correctly" << "\n";
        } else {
            std::cout << "Years not subtracted correctly" << "\n";
        }

    }

输出：

    Years added correctly
    Years subtracted correctly
    Years subtracted correctly

### 返回值

operator++operator++(int)operator--operator--(int) | 自增或自减 month (`std::chrono::month` 的公开成员函数)
operator+=operator-= | 加或减月数 (`std::chrono::month` 的公开成员函数)