NAME¶

std::expected::transform - std::expected::transform

Synopsis¶

template< class F > (1) (since C++23)
constexpr auto transform( F&& f ) &;
template< class F > (2) (since C++23)
constexpr auto transform( F&& f ) const&;
template< class F > (3) (since C++23)
constexpr auto transform( F&& f ) &&;
template< class F > (4) (since C++23)
constexpr auto transform( F&& f ) const&&;

If *this contains an expected value, invokes f and returns a std::expected object
that contains its result; otherwise, returns a std::expected object that contains a
copy of error().

If T is not (possibly cv-qualified) void, the contained value (obtained from
operator*) is passed as an argument to f; otherwise f takes no argument.

1,2) Given type U as
* std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qualified) void,
or
* std::remove_cv_t<std::invoke_result_t<F, decltype(**this)>> otherwise,
and expression /* invoke-expr */ as
* std::invoke(std::forward<F>(f)) if T is (possibly cv-qualified) void, or
* std::invoke(std::forward<F>(f), **this) otherwise.
If U is not a valid value type for std::expected, or U u(/* invoke-expr */); is
ill-formed when std::is_void_v<U> is false, the program is ill-formed.
The effect is equivalent to

if (has_value())
{
if constexpr (std::is_void_v<U>)
{
/* invoke-expr */;
return std::expected<U, E>();
}
else
// the returned std::expected object contains an expected value,
// which is direct-non-list-initialized with /* invoke-expr */
return /* an std::expected<U, E> object */;
}
else
return std::expected<U, E>(std::unexpect, error());

These overloads participate in overload resolution only if
std::is_constructible_v<E, decltype(error())> is true.
3,4) Given type U as
* std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qualified) void,
or
* std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(**this))>>
otherwise,
and expression /* invoke-expr */ as
* std::invoke(std::forward<F>(f)) if T is (possibly cv-qualified) void, or
* std::invoke(std::forward<F>(f), std::move(**this)) otherwise.
If U is not a valid value type for std::expected, or U u(/* invoke-expr */); is
ill-formed when std::is_void_v<U> is false, the program is ill-formed.
The effect is equivalent to

if (has_value())
{
if constexpr (std::is_void_v<U>)
{
/* invoke-expr */;
return std::expected<U, E>();
}
else
// the returned std::expected object contains an expected value,
// which is direct-non-list-initialized with /* invoke-expr */
return /* an std::expected<U, E> object */;
}
else
return std::expected<U, E>(std::unexpect, std::move(error()));

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¶

A std::expected object containing either the result of f or an error value, as
described above.

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 transform was ill-formed if T is not (possibly made well-formed
cv-qualified) void and E is not copyable

See also¶

returns the expected itself if it contains an expected value;
transform_error otherwise, returns an expected containing the transformed unexpected
value
(public member function)

Category:¶

* Todo no example