std::expected<T,E>::operator=

constexpr expected& operator=( const expected& other );

constexpr expected& operator=( expected&& other ) noexcept(/*see below*/);

template< class U = T >
constexpr expected& operator=( U&& v );

template< class G >
constexpr expected& operator=( const unexpected<G>& other );

template< class G >
constexpr expected& operator=( unexpected<G>&& other );

Assigns a new value to an existing expected object.

• If this->has_value() equals other.has_value(), assigns the value contained in other. Does nothing if T is (possibly cv-qualified) void and other.has_value() is true.
• Otherwise, destroys the currently contained value (does nothing if this->has_value() is true and T is (possibly cv-qualified) void), and makes *this contain a copy of the value contained in other.

If no exception was thrown, after assignment, has_value() is equal to other.has_value().

• Overload (1) is defined as deleted unless
  • either T is (possibly cv-qualified) void or std::is_copy_assignable_v<T> is true, and
  • either T is (possibly cv-qualified) void or std::is_copy_constructible_v<T> is true, and
  • std::is_copy_assignable_v<E> is true, and
  • std::is_copy_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>
• Overload (2) participates in overload resolution only if
  • either T is (possibly cv-qualified) void or std::is_move_assignable_v<T> is true, and
  • either T is (possibly cv-qualified) void or std::is_move_constructible_v<T> is true, and
  • std::is_move_assignable_v<E> 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>

• If this->has_value() is true, equivalent to **this = std::forward<U>(v).
• Otherwise, destroys the value contained in *this, and makes *this contain a value initialized from std::forward<U>(v). If an exception is thrown, the old value is retained; *this does not become valueless.

If no exception was thrown, after assignment, this->has_value() is true.

• This overload participates in overload resolution only if
  • std::is_same_v<expected, std::remove_cvref_t<U>> is false, and
  • std::remove_cvref_t<U> is not a specialization of std::unexpected, and
  • std::is_constructible_v<T, U> is true, and
  • std::is_assignable_v<T&, U> is true, and
  • at least one of the following is true:
    • std::is_nothrow_constructible_v<T, U>
    • std::is_nothrow_move_constructible_v<T>
    • std::is_nothrow_move_constructible_v<E>

Let GF be const G& for overload (4), and G for overload (5).

• If this->has_value() is true, destroys the value contained in *this (does nothing if T is (possibly cv-qualified) void), and makes *this contain a value initialized from std::forward<GF>(e.error()). If an exception is thrown, the old value is retained; *this does not become valueless.
• Otherwise, equivalent to this->error() = std::forward<GF>(e.error()).

If no exception was thrown, after assignment, this->has_value() is false.

• This overload participates in overload resolution only if
  • std::is_constructible_v<E, GF> is true, and
  • std::is_assignable_v<E&, GF> is true, and
  • at least one of the following is true:
    • T is (possibly cv-qualified) void
    • std::is_nothrow_constructible_v<E, GF>
    • std::is_nothrow_move_constructible_v<T>
    • std::is_nothrow_move_constructible_v<E>

In all cases, if T is not (possibly cv-qualified) void, the destruction of old value and construction of new value is performed as if by the following exposition-only function reinit_expected.

template<class NewType, class OldType, class... Args>
constexpr void reinit_expected(NewType& new_val, OldType& old_val, Args&&... args)
{
    if constexpr (std::is_nothrow_constructible_v<NewType, Args...>)
    {
        std::destroy_at(std::addressof(old_val));
        std::construct_at(std::addressof(new_val), std::forward<Args>(args)...);
    }
    else if constexpr (std::is_nothrow_move_constructible_v<NewType>)
    {
        NewType temp(std::forward<Args>(args)...);
        std::destroy_at(std::addressof(old_val));
        std::construct_at(std::addressof(new_val), std::move(temp));
    }
    else
    {
        OldType temp(std::move(old_val));
        std::destroy_at(std::addressof(old_val));
        try
        {
            std::construct_at(std::addressof(new_val), std::forward<Args>(args)...);
        }
        catch (...)
        {
            std::construct_at(std::addressof(old_val), std::move(temp));
            throw;
        }
    }
}

Parameters

Return value

*this

Exceptions

Example

See also