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

 

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std::expected

Member functions

expected::expected

expected::~expected

expected::operator=

Observers

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

constexpr expected& operator=( const expected& other );	(1)	(since 哋它亢++23)
constexpr expected& operator=( expected&& other ) noexcept(/*see below*/);	(2)	(since 哋它亢++23)
template< class U = T > constexpr expected& operator=( U&& v );	(3)	(since 哋它亢++23) (T is not cv void)
template< class G > constexpr expected& operator=( const unexpected<G>& other );	(4)	(since 哋它亢++23)
template< class G > constexpr expected& operator=( unexpected<G>&& other );	(5)	(since 哋它亢++23)

Assigns a new value to an existing expected object.

1,2) Assigns the state of other.

• 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 other.has_value() is true and T is (possibly cv-qualified) void, does not construct the new value. Otherwise, the new value is copy-constructed (1) or move-constructed (2) from *other or other.error(), as appropriate. If an exception is thrown, the old value is retained; *this does not become valueless.

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>

3) Assigns from expected value.

• 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>

4,5) Assigns from unexpected value.

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

other	-	another expected object whose contained value to assign
value	-	value to assign to the contained value
e	-	std::unexpected object whose contained value to assign

Return value

*this

Exceptions

1) Throws any exception thrown by the copy constructor or copy assignment operator of T or E.

2) If T is (possibly cv-qualified) void,

noexcept specification:  

noexcept(std::is_nothrow_move_constructible_v<E> && std::is_nothrow_move_assignable_v<E>)

Otherwise,

noexcept specification:  

noexcept(

    std::is_nothrow_move_constructible_v<T> && std::is_nothrow_move_assignable_v<T> &&

    std::is_nothrow_move_constructible_v<E> && std::is_nothrow_move_assignable_v<E>)

3) Throws any exception thrown by the constructor or assignment operator of T.

4,5) Throws any exception thrown by the constructor or assignment operator of E.

See also

emplace

constructs the expected value in-place (public member function)