NAME¶

std::expected::expected - std::expected::expected

Synopsis¶

constexpr expected(); (1) (since C++23)
constexpr expected( const expected& other ); (2) (since C++23)
constexpr expected( expected&& other ) noexcept(/* see below (3) (since C++23)
*/);
template< class U, class G >
constexpr explicit(/* see below */) expected( const expected<U, (4) (since C++23)
G>& other );
template< class U, class G >
constexpr explicit(/* see below */) expected( expected<U, G>&& (5) (since C++23)
other );
template< class U = T > (since C++23)
constexpr explicit(!std::is_convertible_v<U, T>) expected( U&& (6) (T is not cv
v ); void)
template< class G >

constexpr explicit(!std::is_convertible_v<const G&, E>) (7) (since C++23)

expected( const std::unexpected<G>& e );
template< class G >

constexpr explicit(!std::is_convertible_v<G, E>) (8) (since C++23)

expected( std::unexpected<G>&& e );
template< class... Args > (since C++23)
constexpr explicit expected( std::in_place_t, Args&&... args ); (9) (T is not cv
void)
template< class U, class... Args >
(since C++23)
constexpr explicit expected( std::in_place_t, (10) (T is not cv
void)
std::initializer_list<U> il,
Args&&... args );
constexpr explicit expected( std::in_place_t ) noexcept; (11) (since C++23)
(T is cv void)
template< class... Args > (12) (since C++23)
constexpr explicit expected( std::unexpect_t, Args&&... args );
template< class U, class... Args >

constexpr explicit expected( std::unexpect_t, (13) (since C++23)

std::initializer_list<U> il,
Args&&... args );

Constructs a new expected object.

1) Default constructor. If T is not (possibly cv-qualified) void, constructs an
object that contains an expected value, which is value-initialized.
After construction, has_value() returns true.
This overload participates in overload resolution only if T is (possibly
cv-qualified) void or std::is_default_constructible_v<T> is true.
2) Copy constructor. If other.has_value() is false, the new object contains an
unexpected value, which is direct-initialized from other.error(). Otherwise, if T is
not (possibly cv-qualified) void, the new object contains an expected value, which
is direct-initialized from *other.
After construction, has_value() is equal to other.has_value().
This constructor is defined as deleted unless
* either T is (possibly cv-qualified) void, or std::is_copy_constructible_v<T> is
true, and
* std::is_copy_constructible_v<E> is true.
This constructor is trivial if
* either T is (possibly cv-qualified) void, or
std::is_trivially_copy_constructible_v<T> is true, and
* std::is_trivially_copy_constructible_v<E> is true.
3) Move constructor. If other.has_value() is false, the new object contains an
unexpected value, which is direct-initialized from std::move(other.error()).
Otherwise, if T is not (possibly cv-qualified) void, the new object contains an
expected value, which is direct-initialized from std::move(*other).
After construction, has_value() is equal to other.has_value().
This constructor participates in overload resolution only if
* 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.
This constructor is trivial if
* std::is_trivially_move_constructible_v<T> is true, and
* std::is_trivially_move_constructible_v<E> is true.
4,5) Let
* UF be std::add_lvalue_reference_t<const U> for (4) and U for (5), and
* GF be const G& for (4) and G for (5).
If other.has_value() is false, the new object contains an unexpected value, which is
direct-initialized from std::forward<GF>(other.error()). Otherwise, if T is not
(possibly cv-qualified) void, the new object contains an expected value, which is
direct-initialized from std::forward<UF>(*other).
After construction, has_value() is equal to other.has_value().
Each of these constructors does not participate in overload resolution unless the
following conditions are met respectively:
* Either
* T is (possibly cv-qualified) void, and std::is_void_v<U> is true, or
* std::is_constructible_v<T, UF> is true.
* std::is_constructible_v<E, GF> is true.
* If T is not (possibly cv-qualified) bool, T is not constructible or convertible
from any expression of type (possibly const) std::expected<U, G>, i.e., the
following 8 values are all false:
* std::is_constructible_v<T, std::expected<U, G>&
* std::is_constructible_v<T, std::expected<U, G>
* std::is_constructible_v<T, const std::expected<U, G>&
* std::is_constructible_v<T, const std::expected<U, G>
* std::is_convertible_v<std::expected<U, G>&, T>
* std::is_convertible_v<std::expected<U, G>, T>
* std::is_convertible_v<const std::expected<U, G>&, T>
* std::is_convertible_v<const std::expected<U, G>, T>
* std::unexpected<E> is not constructible from any expression of type (possibly
const) std::expected<U, G>, i.e., the following 4 values are all false:
* std::is_constructible_v<std::unexpected<E>, std::expected<U, G>&
* std::is_constructible_v<std::unexpected<E>, std::expected<U, G>
* std::is_constructible_v<std::unexpected<E>, const std::expected<U, G>&
* std::is_constructible_v<std::unexpected<E>, const std::expected<U, G>
These constructors are explicit if std::is_convertible_v<UF, T> or
std::is_convertible_v<GF, E> is false.
6) Constructs an object that contains an expected value, initialized as if
direct-initializing (but not direct-list-initializing) an object of type T with the
expression std::forward<U>(v).
After construction, has_value() returns true.
This constructor does not participate in overload resolution unless the following
conditions are met:
* T is not (possibly cv-qualified) void.
* std::is_same_v<std::remove_cvref_t<U>, std::in_place_t> is false.
* std::is_same_v<expected, std::remove_cvref_t<U>> is false.
* std::is_constructible_v<T, U> is true.
* std::remove_cvref_t<U> is not a specialization of std::unexpected.
* If T is (possibly cv-qualified) bool, std::remove_cvref_t<U> is not a
specialization of std::expected.
7,8) Let GF be const G& for (7) and G for (8). Constructs an object that contains an
unexpected value, which is direct-initialized from std::forward<GF>(e.error()).
After construction, has_value() returns false.
These overloads participate in overload resolution only if
std::is_constructible_v<E, GF> is true.
9) Constructs an object that contains an expected value, which is direct-initialized
from the arguments std::forward<Args>(args)....
After construction, has_value() returns true.
This overload participates in overload resolution only if std::is_constructible_v<T,
Args...> is true.
10) Constructs an object that contains an expected value, which is
direct-initialized from the arguments il, std::forward<Args>(args)....
After construction, has_value() returns true.
This overload participates in overload resolution only if std::is_constructible_v<T,
std::initializer_list<U>&, Args...> is true.
11) Constructs an object such that after construction, has_value() returns true.
12) Constructs an object that contains an unexpected value, which is
direct-initialized from the arguments std::forward<Args>(args)....
After construction, has_value() returns false.
This overload participates in overload resolution only if std::is_constructible_v<E,
Args...> is true.
13) Constructs an object that contains an unexpected value, which is
direct-initialized from the arguments il, std::forward<Args>(args)....
After construction, has_value() returns false.
This overload participates in overload resolution only if std::is_constructible_v<E,
std::initializer_list<U>&, Args...> is true.

Parameters¶

other - another expected object whose contained value is copied
e - std::unexpected object whose contained value is copied
v - value with which to initialize the contained value
args... - arguments with which to initialize the contained value
il - initializer list with which to initialize the contained value

Exceptions¶

1) Throws any exception thrown by the constructor of T.
If T is (possibly cv-qualified) void,
noexcept specification:
noexcept

2) Throws any exception thrown by the constructor of T or E.
3) If T is (possibly cv-qualified) void,
noexcept specification:
noexcept(std::is_nothrow_move_constructible_v<E>)
Otherwise,
noexcept specification:
noexcept(std::is_nothrow_move_constructible_v<T>
&& std::is_nothrow_move_constructible_v<E>)
4,5) Throws any exception thrown by the constructor of T or E.
6) Throws any exception thrown by the constructor of T.
7,8) Throws any exception thrown by the constructor of E.
9,10) Throws any exception thrown by the constructor of T.
12,13) Throws any exception thrown by the constructor of E.

Example¶

This section is incomplete
Reason: no example

See also¶

unexpected represented as an unexpected value
(C++23) (class template)
in_place
in_place_type
in_place_index in-place construction tag
in_place_t (tag)
in_place_type_t
in_place_index_t
(C++17)
unexpect in-place construction tag for unexpected value in expected
unexpect_t (tag)
(C++23)

Categories:¶

* unconditionally noexcept
* conditionally noexcept
* Todo no example