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

 

C++

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Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

std::expected

Member functions

expected::expected (C++23)

expected::~expected (C++23)

expected::operator= (C++23)

Observers

expected::operator->expected::operator* (C++23)(C++23)

expected::operator boolexpected::has_value (C++23)(C++23)

expected::value (C++23)

expected::error (C++23)

expected::value_or (C++23)

Monadic operations

expected::and_then (C++23)

expected::or_else (C++23)

expected::transform (C++23)

expected::transform_error (C++23)

Modifiers

expected::emplace (C++23)

expected::swap (C++23)

Non-member functions

operator== (C++23)

swap (C++23)

Helper classes

unexpected (C++23)

bad_expected_access (C++23)

unexpect_tunexpect (C++23)(C++23)

 

constexpr expected() noexcept(/* see below */);	(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, G>& other );	(4)	(since C++23)
template< class U, class G > constexpr explicit(/* see below */) expected( expected<U, G>&& other );	(5)	(since C++23)
template< class U = T > constexpr explicit(!std::is_convertible_v<U, T>) expected( U&& v );	(6)	(since C++23) (T is not cv void)
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) (T is not cv void)
template< class U, class... Args > constexpr explicit expected( std::in_place_t,                              std::initializer_list<U> il, Args&&... args );	(10)	(since C++23) (T is not cv void)
template< class... Args > constexpr explicit expected( std::in_place_t ) noexcept;	(11)	(since C++23) (T is cv void)
template< class... Args > constexpr explicit expected( std::unexpect_t, Args&&... args );	(12)	(since C++23)
template< class U, class... Args > constexpr explicit expected( std::unexpect_t,                              std::initializer_list<U> il, Args&&... args );	(13)	(since C++23)

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.

See also

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