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

constexpr void swap( expected& other ) noexcept(/*see below*/);

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

Return value

(none).

Exceptions

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.

Example

See also