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

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Utilities library

Language support

Type support (basic types, RTTI)

Library feature-test macros(C++20)

Program utilities

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initializer_list

(C++11)

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(C++20)

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(C++26)

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(C++20)

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three_way_comparablethree_way_comparable_with

(C++20)(C++20)

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(C++20)

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(C++20)

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(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)

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(C++20)(C++20)(C++20)

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(C++20)(C++20)(C++20)

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

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cmp_equalcmp_lesscmp_less_than

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cmp_not_equalcmp_greatercmp_greater_than

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in_range

(C++20)

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tuple_size (C++11)
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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
template<class U=std::remove_cv_t<T>> constexpr T value_or( U&& default_value)const&;	(1)	(since C++23)
template<class U=std::remove_cv_t<T>> constexpr T value_or( U&& default_value)&&;	(2)	(since C++23)

Returns the expected value if it exists, otherwise returnsdefault_value.

Thevoid partial specialization does not have these member functions.

1) Ifstd::is_copy_constructible_v<T> orstd::is_convertible_v<U, T> isfalse, the program is ill-formed.

2) Ifstd::is_move_constructible_v<T> orstd::is_convertible_v<U, T> isfalse, the program is ill-formed.

[edit]Parameters

default_value

the value to use in case*this does not contain an expected value

[edit]Return value

1)has_value()?**this:static_cast<T>(std::forward<U>(default_value))

2)has_value()? std::move(**this):static_cast<T>(std::forward<U>(default_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	`U` does not have a default template argument	specified

[edit]See also

value	returns the expected value (public member function)[edit]
error_or	returns the unexpected value if present, another value otherwise (public member function)[edit]