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

 

C++

Compiler support

Freestanding and hosted

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

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 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 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) to_chars_result (C++17) from_chars_result (C++17)

 

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

 

Primary template
constexpr T& value() &;	(1)	(since C++23)
constexpr const T& value() const&;	(2)	(since C++23)
constexpr T&& value() &&;	(3)	(since C++23)
constexpr const T&& value() const&&;	(4)	(since C++23)
void partial specialization
constexpr void value() const&;	(5)	(since C++23)
constexpr void value() &&;	(6)	(since C++23)

If *this contains an expected value, returns a reference to the contained value. Returns nothing for void partial specialization.

Otherwise, throws an exception of type std::bad_expected_access<std::decay_t<E>> that contains a copy of error().

1,2) If std::is_copy_constructible_v<E> is false, the program is ill-formed.

3,4) If std::is_copy_constructible_v<E> or std::is_constructible_v<E, decltype(std::move(error()))> is false, the program is ill-formed.

5) If std::is_copy_constructible_v<E> is false, the program is ill-formed.

6) If std::is_move_constructible_v<E> is false, the program is ill-formed.

Return value

1,2) val

3,4) std::move(val)

Exceptions

1,2,5) Throws std::bad_expected_access(std::as_const(error())) if *this contains an unexpected value.

3,4,6) Throws std::bad_expected_access(std::move(error())) if *this contains an unexpected value.

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 3940	C++23	E was not required to be copy/move-constructible for overloads (5,6)	required

See also

value_or	returns the expected value if present, another value otherwise (public member function)
operator->operator*	accesses the expected value (public member function)
error	returns the unexpected value (public member function)
bad_expected_access (C++23)	exception indicating checked access to an expected that contains an unexpected value (class template)