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, and the following corresponding declaration is ill-formed:
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 object whose call signature returns a non-reference type |
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> (std::unexpect, std::move(error()))
| |
| (5,6) | std::expected<U, E>(std::unexpect, error()) | |
| (7,8) | std::expected<U, E> (std::unexpect, std::move(error()))
| |
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*does not. - ↑ **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) |