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

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

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Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics 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
template< class F > constexpr auto or_else( F&& f ) &;	(1)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) const&;	(2)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) &&;	(3)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&;	(4)	(since C++23)
void partial specialization
template< class F > constexpr auto or_else( F&& f ) &;	(5)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) const&;	(6)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) &&;	(7)	(since C++23)
template< class F > constexpr auto or_else( F&& f ) const&&;	(8)	(since C++23)

If *this contains an unexpected value, invokes f with the unexpected value of *this as the argument and returns its result. Otherwise, returns a std::expected object that represents an expected value.

1-4) The expected value is initialized with the expected value val of *this.

Given type G as:

1,2) std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>

3,4) std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>

5,6) std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>

7,8) std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>

If G is not a specialization of std::expected, or std::is_same_v<G::value_type, T> is false, the program is ill-formed.

1,2) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype((val))> is true.

3,4) These overloads participate in overload resolution only if std::is_constructible_v<T, decltype(std::move(val))> is true.

Parameters

f

-

a suitable function or Callable object that returns a std::expected

Return value

Overload	Value of has_value()
	true	false
(1,2)	G(std::in_place, val)	std::invoke(std::forward<F>(f), error())
(3,4)	G(std::in_place, std::move(val))	std::invoke(std::forward<F>(f), std::move(error()))
(5,6)	G()	std::invoke(std::forward<F>(f), error())
(7,8)		std::invoke(std::forward<F>(f), std::move(error()))

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_expected	202211L	(C++23)	Monadic functions for std::expected

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 3938	C++23	the expected value was obtained by value()	changed to **this
LWG 3973	C++23	the expected value was obtained by **this	changed to val

See also

transform_error

returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function)