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

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< 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 std::is_constructible_v<E, std::initializer_list<U>&, Args...> is true.

[edit] Exceptions

[edit] Primary template constructors

1) Throws any exception thrown by the initialization of the expected value.
2) Throws any exception thrown by the initialization of the expected or unexpected value.
3) Throws any exception thrown by the initialization of the expected or unexpected value.
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 initialization of the expected or unexpected value.
6) Throws any exception thrown by the initialization of the expected value.
7,8) Throws any exception thrown by the initialization of the unexpected value.
9,10) Throws any exception thrown by the initialization of the expected value.
11,12) Throws any exception thrown by the initialization of the unexpected value.

[edit] void partial specialization constructors

14-19) Throws any exception thrown by the initialization of the unexpected value.
21,22) Throws any exception thrown by the initialization of the unexpected value.

[edit] Example

This section is incomplete
Reason: no example

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3886 C++23 the default template argument of overload (6) was T changed to std::remove_cv_t<T>

[edit] See also

(C++23)
 represented as an unexpected value
(class template) [edit]
(C++17)
 in-place construction tag
(tag)[edit]
(C++23)
 in-place construction tag for unexpected value in expected
(tag)[edit]
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