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

 

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External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison 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_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (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)

 

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

expected::error_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==(std::expected)

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

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, 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 = std::remove_cv_t<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)
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.

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>

See also

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