Namespaces
Variants

    std::expected<T,E>::swap

    来自 zh.cppreference.net
     
    C++
     
    工具库
    通用工具
    关系运算符 (C++20 中弃用)
    整数比较函数
    (C++20)(C++20)(C++20)    
    (C++20)
    交换 与类型操作
    (C++20)
    (C++14)
    (C++11)
    (C++23)
    (C++11)
    (C++23)
    (C++11)
    (C++11)
    (C++17)
    常用词汇类型
    (C++11)
    (C++17)
    (C++17)
    (C++17)
    (C++11)
    (C++11)
    (C++17)
    (C++17)
    (C++23)



     
    std::expected
     
    主模板
    		 
    constexpr void swap( expected& other ) noexcept(/* see below */);
    		(1) (C++23 起)
    void 部分特化
    		 
    constexpr void swap( expected& other ) noexcept(/* see below */);
    		(2) (C++23 起)

    将内容与 other 的内容交换。

    1) 按如下方式交换所含的值：
    的值
     has_value()      		的值 other.has_value()
    true false
    true using std::swap;
    swap(val, rhs.val);         		见下
    false other.swap(*this); using std::swap;
    swap(unex, rhs.unex);
    如果 has_value() 为 true 且 other.has_value() 为 false，则等价于：

    // Case 1: the move constructions of unexpected values are non-throwing:
    // “other.unex” will be restored if the construction of “other.val” fails
    if constexpr (std::is_nothrow_move_constructible_v<E>)
    {
        E temp(std::move(other.unex));
        std::destroy_at(std::addressof(other.unex));
        try
        {
            std::construct_at(std::addressof(other.val), std::move(val)); // may throw
            std::destroy_at(std::addressof(val));
            std::construct_at(std::addressof(unex), std::move(temp));
        }
        catch(...)
        {
            std::construct_at(std::addressof(other.unex), std::move(temp));
            throw;
        }
    }
    // Case 2: the move constructions of expected values are non-throwing:
    // “this->val” will be restored if the construction of “this->unex” fails
    else
    {
        T temp(std::move(val));
        std::destroy_at(std::addressof(val));
        try
        {
            std::construct_at(std::addressof(unex), std::move(other.unex)); // may throw 
            std::destroy_at(std::addressof(other.unex));
            std::construct_at(std::addressof(other.val), std::move(temp));
        }
        catch(...)
        {
            std::construct_at(std::addressof(val), std::move(temp));
            throw;
        }
    }
    has_val = false;
    rhs.has_val = true;

    仅当以下所有值均为 true 时，此重载才参与重载决议：
    • std::is_swappable_v<T>
    • std::is_swappable_v<E>
    • std::is_move_constructible_v<T> && std::is_move_constructible_v<E>
    • std::is_nothrow_move_constructible_v<T> || std::is_nothrow_move_constructible_v<E>
    2) 按如下方式交换非预期值：
    的值
     has_value()      		的值 other.has_value()
    true false
    true using std::swap;
    swap(val, rhs.val);         		std::construct_at(std::addressof(unex),
                      std::move(rhs.unex));
    std::destroy_at(std::addressof(rhs.unex));
    has_val = false;
    rhs.has_val = true;
    false other.swap(*this); using std::swap;
    swap(unex, rhs.unex);
    仅当 std::is_swappable_v<E> 和 std::is_move_constructible_v<E> 均为 true 时，此重载才参与重载决议。

    参数

    其他 - 用于交换内容的 expected 对象

    异常

    1)
    noexcept 规范：
    noexcept (

    std:: is_nothrow_move_constructible_v < T > && std:: is_nothrow_swappable_v < T > &&
    std:: is_nothrow_move_constructible_v < E > && std:: is_nothrow_swappable_v < E >

    )
    2)
    noexcept 规范：
    noexcept (

    std:: is_nothrow_move_constructible_v < E > && std:: is_nothrow_swappable_v < E >

    )

    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{CAT FACE}");
    Ex ex2{"\N{GREEN HEART}"};
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2, "\n\n");
    ex2 = std::unexpected(13);
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2, "\n\n");
    ex2 = std::unexpected(37);
    show(ex1, ex2, "after ex1.swap(ex2):\n");
    ex1.swap(ex2);
    show(ex1, ex2);
}

    输出： 

    ex1.value() = 🐱  ex2.value() = 💚  after ex1.swap(ex2):
ex1.value() = 💚  ex2.value() = 🐱 
ex1.value() = 💚  ex2.error() = 13  after ex1.swap(ex2):
ex1.error() = 13  ex2.value() = 💚 
ex1.error() = 13  ex2.error() = 37  after ex1.swap(ex2):
ex1.error() = 37  ex2.error() = 13

    参见

    (C++23)
    		特化 std::swap 算法
    (函数)