std::expected<T,E>::transform
From cppreference.com
Primary template |
||
|
template
<
class F >
constexpr auto transform( F&& f ) & ; |
(1) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) const & ; |
(2) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) && ; |
(3) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) const && ; |
(4) | (since C++23) |
void partial specialization |
||
|
template
<
class F >
constexpr auto transform( F&& f ) & ; |
(5) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) const & ; |
(6) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) && ; |
(7) | (since C++23) |
|
template
<
class F >
constexpr auto transform( F&& f ) const && ; |
(8) | (since C++23) |
If *this represents an expected value, invokes f and returns a std::expected object that contains an expected value, which is initialized with its result (or value-initialized if the result type is void). Otherwise, returns a std::expected object that contains an unexpected value, which is initialized with the unexpected value of *this
5-8) f is invoked without any argument.
Given type U as:
5-8)
std::remove_cv_t
<
std::invoke_result_t
<F>>
If any of the following conditions is satisfied, the program is ill-formed:
Uis not a valid value type forstd::expected.- std::is_void_v<U> is false
1,2)
U u(
std::invoke
(
std::forward
<F>
(f)
val
));
3,4)
U u(
std::invoke
(
std::forward
<F>
(f), std::
move
(
val
)));
5-8)
U u(
std::invoke
(
std::forward
<F>
(f)
)
)
;
1,2) These overloads participate in overload resolution only if
std::is_constructible_v
<E, decltype(error(
)
)
>
is true
3,4) These overloads participate in overload resolution only if
std::is_constructible_v
<E, decltype(std::
move
(error(
)
)
)
>
is true
5,6) These overloads participate in overload resolution only if
std::is_constructible_v
<E, decltype(error(
)
)
>
is true
7,8) These overloads participate in overload resolution only if
std::is_constructible_v
<E, decltype(std::
move
(error(
)
)
)
>
is true
Parameters
| f | - | a suitable function or Callable |
Return value
Given expression expr as:
1,2)
std::invoke
(
std::forward
<F>
(f)
val
)
3,4)
std::invoke
(
std::forward
<F>
(f),std::
move
(
val
))
5-8)
std::invoke
(
std::forward
<F>
(f)
)
The return values are defined as follows:
| Overload | Value of has_value()
| |
|---|---|---|
| true | false | |
| (1,2) |
|
std::expected <U, E> ( std::unexpect, error( ) ) |
| (3,4) |
std::expected<U, E>
|
|
| (5,6) | std::expected <U, E> ( std::unexpect, error( ) ) | |
| (7,8) |
std::expected<U, E>
|
|
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) |