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

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three_way_comparablethree_way_comparable_with

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(C++20)

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(C++20)

(C++20)

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(C++20)

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(C++20)

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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
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expected::and_then
expected::or_else
expected::transform
expected::transform_error
Modifiers
expected::emplace
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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 F> constexprauto or_else( F&& f)&;	(1)	(since C++23)
template<class F> constexprauto or_else( F&& f)const&;	(2)	(since C++23)
template<class F> constexprauto or_else( F&& f)&&;	(3)	(since C++23)
template<class F> constexprauto or_else( F&& f)const&&;	(4)	(since C++23)
void partial specialization
template<class F> constexprauto or_else( F&& f)&;	(5)	(since C++23)
template<class F> constexprauto or_else( F&& f)const&;	(6)	(since C++23)
template<class F> constexprauto or_else( F&& f)&&;	(7)	(since C++23)
template<class F> constexprauto or_else( F&& f)const&&;	(8)	(since C++23)

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

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

Given typeG 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()))>>

IfG is not a specialization ofstd::expected, orstd::is_same_v<G::value_type, T> isfalse, the program is ill-formed.

1,2) These overloads participate in overload resolution only ifstd::is_constructible_v<T, decltype((val))> istrue.

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

[edit]Parameters

f - a suitable function orCallable object that returns astd::expected

[edit]Return value

Overload	Value of`has_value()`
Overload	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)	G()	std::invoke(std::forward<F>(f), std::move(error()))

[edit]Notes

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

[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 3938	C++23	the expected value was obtained byvalue()^[1]	changed to**this
LWG 3973	C++23	the expected value was obtained by**this^[2]	changed to`val`

↑value() requiresE to be copy constructible (seeLWG issue 3843), whereoperator* does not.
↑**this can triggerargument-dependent lookup.

[edit]See also

transform_error

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

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