std::expected<T,E>::or_else
From cppreference.com
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
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
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
|
Example
| This section is incomplete Reason: no example |
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* - ↑ **this can trigger argument-dependent lookup.
See also
returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function) |