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    swap(std::expected)

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    std::expected
     
    friend constexpr void swap( expected& lhs, expected& rhs ) noexcept(/*see below*/);
    		(C++23 起)

    为重载 std::swap 算法针对 std::expected 提供重载。交换 lhs 与 rhs 的状态。实际效果为调用 lhs.swap(rhs)。

    此重载仅在 lhs.swap(rhs) 有效时参与重载决议。

    此函数 对普通的 无限定 或 有限定查找 不可见，只能通过 实参依赖查找 在 std::expected<T, E> 是实参关联类时被找到。

    参数

    lhs, rhs - 要交换状态的 expected 对象

    返回值

    (无)

    异常

    noexcept 规范：
    noexcept ( noexcept ( lhs. swap ( rhs ) ) )

    示例

    运行此代码 
    #include <expected>
#include <iostream>
#include <string_view>
using Ex = std::expected<std::string, int>;
void show(const Ex& ex1, const Ex& ex2, std::string_view term = "\n")
{
    for (int i{}; i != 2; ++i)
    {
        std::cout << (i ? "ex2" : "ex1");
        if (const Ex& ex = (i ? ex2 : ex1); ex.has_value())
            std::cout << ".value() = " << *ex << "  ";
        else
            std::cout << ".error() = " << ex.error() << "  ";
    }
    std::cout << term;
}
int main()
{
    Ex ex1("\N{SMILING FACE WITH SUNGLASSES}");
    Ex ex2{"\N{SLIGHTLY SMILING FACE}"};
    show(ex1, ex2, "after swap(ex1, ex2):\n");
    std::swap(ex1, ex2);
    show(ex1, ex2, "\n\n");
    ex2 = std::unexpected(13);
    show(ex1, ex2, "after swap(ex1, ex2):\n");
    std::swap(ex1, ex2);
    show(ex1, ex2, "\n\n");
    ex2 = std::unexpected(37);
    show(ex1, ex2, "after swap(ex1, ex2):\n");
    std::swap(ex1, ex2);
    show(ex1, ex2);
}

    输出： 

    ex1.value() = 😎  ex2.value() = 🙂  after swap(ex1, ex2):
ex1.value() = 🙂  ex2.value() = 😎  
ex1.value() = 🙂  ex2.error() = 13  after swap(ex1, ex2):
ex1.error() = 13  ex2.value() = 🙂  
ex1.error() = 13  ex2.error() = 37  after swap(ex1, ex2):
ex1.error() = 37  ex2.error() = 13

    参阅

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