std::expected<T,E>::transform_error
|
(1) | (since C++23) |
|
(2) | (since C++23) |
|
(3) | (since C++23) |
|
(4) | (since C++23) |
If *this contains an error value, invokes f with the argument error() and returns a std::expected object that contains its result; otherwise, returns a std::expected object that contains a copy of the contained expected value (obtained from operator*).
If
G is not a valid template argument for std::unexpected, or G g(std::invoke(std::forward<F>(f), error())); is ill-formed, the program is ill-formed.
The effect is equivalent to
if (has_value())
{
if (std::is_void_v<T>)
return std::expected<T, G>();
else
return std::expected<T, G>(std::in_place, **this);
}
else
// the returned std::expected object contains an unexpected value,
// which is direct-non-list-initialized with
// std::invoke(std::forward<F>(f), error())
return /* an std::expected<T, G> object */;
These overloads participate in overload resolution only if
std::is_void_v<T> or std::is_constructible_v<T, decltype(**this)> is true.
If
G is not a valid template argument for std::unexpected, or G g(std::invoke(std::forward<F>(f), std::move(error()))); is ill-formed, the program is ill-formed.
The effect is equivalent to
These overloads participate in overload resolution only if
if (has_value())
{
if (std::is_void_v<T>)
return std::expected<T, G>();
else
return std::expected<T, G>(std::in_place, std::move(**this));
}
else
// the returned std::expected object contains an unexpected value,
// which is direct-non-list-initialized with
// std::invoke(std::forward<F>(f), std::move(error()))
return /* an std::expected<T, G> object */;std::is_void_v<T> or std::is_constructible_v<T, decltype(std::move(**this))> is true.
Parameters
| f | - | a suitable function or Callable object whose call signature returns a non-reference type |
Return value
A std::expected object containing either the result of f or an expected value, as described above.
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 | transform_error was ill-formed if T is not(possibly cv-qualified) void and E is not copyable |
made well-formed |
See also
returns the expected itself if it contains an expected value; otherwise, returns the result of the given function on the unexpected value (public member function) |
|
returns an expected containing the transformed expected value if it exists; otherwise, returns the expected itself (public member function) |
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