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

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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)

(C++20)

(C++20)

(C++20)

(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

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bitset
hash (C++11)

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rel_ops::operator!=rel_ops::operator>

rel_ops::operator<=rel_ops::operator>=

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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)

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swap
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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)

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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

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

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

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

1,2) Ifstd::is_copy_constructible_v<E> isfalse, the program is ill-formed.

3,4) Ifstd::is_copy_constructible_v<E> orstd::is_constructible_v<E, decltype(std::move(error()))> isfalse, the program is ill-formed.

5) Ifstd::is_copy_constructible_v<E> isfalse, the program is ill-formed.

6) Ifstd::is_move_constructible_v<E> isfalse, the program is ill-formed.

[edit]Return value

1,2)val

3,4)std::move(val)

[edit]Exceptions

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

3,4,6) Throwsstd::bad_expected_access(std::move(error())) if*this contains an 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 3940	C++23	`E` was not required to be copy/move-constructible for overloads(5,6)	required

[edit]See also

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