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

From cppreference.com
< cpp‎ | utility‎ | expected
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::expected
 
Main template
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 expected( const expected<U, G>& other );
 (4) (since C++23)
(conditionally explicit)
template< class U, class G >
constexpr expected( expected<U, G>&& other );
 (5) (since C++23)
(conditionally explicit)
template< class U = std::remove_cv_t<T> >
constexpr explicit(!std::is_convertible_v<U, T>) expected( U&& v );
 (6) (since C++23)
template< class G >

constexpr explicit(!std::is_convertible_v<const G&, E>)

    expected( const std::unexpected<G>& e );
 (7) (since C++23)
template< class G >

constexpr explicit(!std::is_convertible_v<G, E>)

    expected( std::unexpected<G>&& e );
 (8) (since C++23)
template< class... Args >
constexpr explicit expected( std::in_place_t, Args&&... args );
 (9) (since C++23)
template< class U, class... Args >

constexpr explicit
    expected( std::in_place_t,

              std::initializer_list<U> il, Args&&... args );
 (10) (since C++23)
template< class... Args >
constexpr explicit expected( std::unexpect_t, Args&&... args );
 (11) (since C++23)
template< class U, class... Args >

constexpr explicit
    expected( std::unexpect_t,

              std::initializer_list<U> il, Args&&... args );
 (12) (since C++23)
void partial specialization
constexpr expected();
 (13) (since C++23)
constexpr expected( const expected& other );
 (14) (since C++23)
constexpr expected( expected&& other )
    noexcept(std::is_nothrow_move_constructible_v<E>);
 (15) (since C++23)
template< class U, class G >

constexpr explicit(!std::is_convertible_v<const G&, E>)

    expected( const expected<U, G>& other );
 (16) (since C++23)
template< class U, class G >

constexpr explicit(!std::is_convertible_v<G, E>)

    expected( expected<U, G>&& other );
 (17) (since C++23)
template< class G >

constexpr explicit(!std::is_convertible_v<const G&, E>)

    expected( const std::unexpected<G>& e );
 (18) (since C++23)
template< class G >

constexpr explicit(!std::is_convertible_v<G, E>)

    expected( std::unexpected<G>&& e );
 (19) (since C++23)
constexpr explicit expected( std::in_place_t );
 (20) (since C++23)
template< class... Args >
constexpr explicit expected( std::unexpect_t, Args&&... args );
 (21) (since C++23)
template< class U, class... Args >

constexpr explicit
    expected( std::unexpect_t,

              std::initializer_list<U> il, Args&&... args );
 (22) (since C++23)

Constructs a new expected object.

Contents

[edit] 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

[edit] Effects

[edit] Primary template constructors

Overload Initialization
method 		Initializer for... has_value()
after construction
the expected value the unexpected value
(1) Value (empty) - true
(2) Direct
(non-list) 		*other other.error() other.has_value()
  • If true, only initializes the expected value.
  • If false, only initializes the unexpected value.
(3) std::move(*other) std::move(other.error())
(4) std::forward<const U&>
    (*other) 		std::forward<const G&>
    (other.error())
(5) std::forward<U>(*other) std::forward<G>
    (other.error())
(6) std::forward<U>(v) - true
(7) - std::forward<const G&>
    (e.error()) 		false
(8) std::forward<G>(e.error())
(9) std::forward<Args>(args)... - true
(10) il,
std::forward<Args>(args)...
(11) - std::forward<Args>(args)... false
(12) il,
std::forward<Args>(args)...

[edit] void partial specialization constructors

Overload Initialization method Initializer for the unexpected value has_value() after construction
(13) N/A - true
(14) Direct (non-list) rhs.error() other.has_value()
  • If false, initializes the unexpected value.
(15) std::move(rhs.error)
(16) std::forward<const G&>(rhs.error())
(17) std::forward<G>(rhs.error())
(18) std::forward<const G&>(e.error()) false
(19) std::forward<G>(e.error())
(20) N/A - true
(21) Direct (non-list) std::forward<Args>(args)... false
(22) il, std::forward<Args>(args)...

[edit] Constraints and supplement information

[edit] Primary template constructors

1) This overload participates in overload resolution only if std::is_default_constructible_v<T> is true.
2) This constructor is defined as deleted unless std::is_copy_constructible_v<T> and std::is_copy_constructible_v<E> are both true.
This constructor is trivial if std::is_trivially_copy_constructible_v<T> and std::is_trivially_copy_constructible_v<E> are both true.
3) This overload participates in overload resolution only if std::is_move_constructible_v<T> and std::is_move_constructible_v<E> are both true.
This constructor is trivial if std::is_trivially_move_constructible_v<T> and std::is_trivially_move_constructible_v<E> are both true.
4,5) These overloads participate in overload resolution only if all following conditions are satisfied:
4) This constructor is explicit if std::is_convertible_v<const U&, T> or std::is_convertible_v<const G&, E> is false.
5) This constructor is explicit if std::is_convertible_v<U, T> or std::is_convertible_v<G, E> is false.
6) This overload participates in overload resolution only if all following conditions are satisfied:
7) This overload participates in overload resolution only if std::is_constructible_v<E, const G&> is true.
8) This overload participates in overload resolution only if std::is_constructible_v<E, G> is true.
9) This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true.
10) This overload participates in overload resolution only if std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true.
11) This overload participates in overload resolution only if std::is_constructible_v<E, Args...> is true.
12) This overload participates in overload resolution only if std::is_constructible_v<E, std::initializer_list<U>&, Args...> is true.

[edit] void partial specialization constructors

14) This constructor is defined as deleted unless std::is_copy_constructible_v<E> is true.
This constructor is trivial if std::is_trivially_copy_constructible_v<E> is true.
15) This overload participates in overload resolution only if std::is_move_constructible_v<E> is true.
This constructor is trivial if std::is_trivially_move_constructible_v<E> is true.
16,17) These overloads participate in overload resolution only if all following conditions are satisfied:
18) This overload participates in overload resolution only if std::is_constructible_v<E, const G&> is true.
19) This overload participates in overload resolution only if std::is_constructible_v<E, G> is true.
21) This overload participates in overload resolution only if std::is_constructible_v<E, Args...> is true.
22) This overload participates in overload resolution only if