std::expected<T,E>::or_else
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std::expected
expected::or_else
Primary template
template< class F >
constexpr auto or_else( F&& f ) &;
(1)
(since C++23)
constexpr auto or_else( F&& f ) &;
template< class F >
constexpr auto or_else( F&& f ) const&;
(2)
(since C++23)
constexpr auto or_else( F&& f ) const&;
template< class F >
constexpr auto or_else( F&& f ) &&;
(3)
(since C++23)
constexpr auto or_else( F&& f ) &&;
template< class F >
constexpr auto or_else( F&& f ) const&&;
(4)
(since C++23)
constexpr auto or_else( F&& f ) const&&;
void partial specialization
template< class F >
constexpr auto or_else( F&& f ) &;
(5)
(since C++23)
constexpr auto or_else( F&& f ) &;
template< class F >
constexpr auto or_else( F&& f ) const&;
(6)
(since C++23)
constexpr auto or_else( F&& f ) const&;
template< class F >
constexpr auto or_else( F&& f ) &&;
(7)
(since C++23)
constexpr auto or_else( F&& f ) &&;
template< class F >
constexpr auto or_else( F&& f ) const&&;
(8)
(since C++23)
constexpr auto or_else( F&& f ) const&&;
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.
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.[edit] Parameters
f
-
a suitable function or Callable object that returns a
std::expected
[edit] 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())) | |
[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
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 by value()[1] | changed to **this |
| LWG 3973 | C++23 | the expected value was obtained by **this[2] | changed to val
|
- ↑
value()requiresEto be copy constructible (see LWG issue 3843), whereoperator*does not. - ↑ **this can trigger argument-dependent lookup.
[edit] See also
returns the
(public member function) [edit]
expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function) [edit]