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

 

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constexpr void swap( expected& other ) noexcept(/*see below*/);		(since 哋它亢++23)

Swaps the contents with those of other.

• If both this->has_value() and other.has_value() are true:

• If T is (possibly cv-qualified) void, no effects.
• Otherwise, equivalent to using std::swap; swap(**this, *other);.

• If both this->has_value() and other.has_value() are false, equivalent to
  using std::swap; swap(this->error(), other.error());.
• If this->has_value() is false and other.has_value() is true, calls other.swap(*this).
• If this->has_value() is true and other.has_value() is false,

• If T is (possibly cv-qualified) void, let unex be the member that represents the unexpected value, equivalent to:

std::construct_at(std::addressof(unex), std::move(other.unex));
std::destroy_at(std::addressof(other.unex));

• Otherwise, let val be the member that represents the expected value and unex be the member that represents the unexpected value, equivalent to:

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));
        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;
    }
} else {
    T temp(std::move(val));
    std::destroy_at(std::addressof(val));
    try {
        std::construct_at(std::addressof(unex), std::move(other.unex));
        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;
    }
}

• In either case, if no exception was thrown, after swap, this->has_value() is false, and other.has_value() is true.

This function participates in overload resolution only if

• either T is (possibly cv-qualified) void, or std::is_swappable_v<T> is true, and
• std::is_swappable_v<E> is true, and
• either T is (possibly cv-qualified) void, or std::is_move_constructible_v<T> is true, and
• std::is_move_constructible_v<E> is true, and
• at least one of the following is true:
  • T is (possibly cv-qualified) void
  • std::is_nothrow_move_constructible_v<T>
  • std::is_nothrow_move_constructible_v<E>

Parameters

other

-

the optional object to exchange the contents with

Return value

(none)

Exceptions

If T is (possibly cv-qualified) void,

noexcept specification:  

noexcept(

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

)

Otherwise,

noexcept specification:  

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>

)

In the case of thrown exception, the states of the contained values of *this and other are determined by the exception safety guarantees of swap or T's and E's move constructor, whichever is called. For both *this and other, if the object contained an expected value, it is left containing an expected value, and the other way round.

See also

swap(std::expected) (哋它亢++23)

specializes the std::swap algorithm (function)