std::expected<T,E>::transform_error

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Utilities library

Language support

Type support (basic types, RTTI)

Library feature-test macros(C++20)

Program utilities

Variadic functions

initializer_list

(C++11)

is_constant_evaluated

(C++20)

is_within_lifetime

(C++26)

source_location

(C++20)

Coroutine support(C++20)

Contract support(C++26)

Three-way comparison

three_way_comparablethree_way_comparable_with

(C++20)(C++20)

strong_ordering

(C++20)

weak_ordering

(C++20)

partial_ordering

(C++20)

common_comparison_category

(C++20)

compare_three_way_result

(C++20)

(C++20)

(C++20)

(C++20)

(C++20)

compare_strong_order_fallback

(C++20)

compare_weak_order_fallback

(C++20)

compare_partial_order_fallback

(C++20)

is_eqis_ltis_lteq

(C++20)(C++20)(C++20)

is_neqis_gtis_gteq

(C++20)(C++20)(C++20)

General utilities

Function objects
Bit manipulation(C++20)
C-style bit manipulation(C++26)
bitset
hash (C++11)

Relational operators(deprecated in C++20)

rel_ops::operator!=rel_ops::operator>

rel_ops::operator<=rel_ops::operator>=

Integer comparison functions

cmp_equalcmp_lesscmp_less_than

(C++20)(C++20)(C++20)

cmp_not_equalcmp_greatercmp_greater_than

(C++20)(C++20)(C++20)

in_range

(C++20)

Swap andtype operations

swap
ranges::swap (C++20)
exchange (C++14)
declval (C++11)
to_underlying (C++23)

forward (C++11)
forward_like (C++23)
move (C++11)
move_if_noexcept (C++11)
as_const (C++17)

Common vocabulary types

pair
tuple (C++11)
optional (C++17)
any (C++17)
variant (C++17)

tuple_size (C++11)
tuple_element (C++11)
apply (C++17)
make_from_tuple (C++17)
expected (C++23)

[edit]

std::expected

Member functions
expected::expected
expected::~expected
expected::operator=
Observers
expected::operator->expected::operator*
expected::operator boolexpected::has_value
expected::value
expected::error
expected::value_or
expected::error_or
Monadic operations
expected::and_then
expected::or_else
expected::transform
expected::transform_error
Modifiers
expected::emplace
expected::swap
Non-member functions
operator==(std::expected)
swap(std::expected)
Helper classes
unexpected
bad_expected_access
unexpect_tunexpect

[edit]

Primary template
template<class F> constexprauto transform_error( F&& f)&;	(1)	(since C++23)
template<class F> constexprauto transform_error( F&& f)const&;	(2)	(since C++23)
template<class F> constexprauto transform_error( F&& f)&&;	(3)	(since C++23)
template<class F> constexprauto transform_error( F&& f)const&&;	(4)	(since C++23)
void partial specialization
template<class F> constexprauto transform_error( F&& f)&;	(5)	(since C++23)
template<class F> constexprauto transform_error( F&& f)const&;	(6)	(since C++23)
template<class F> constexprauto transform_error( F&& f)&&;	(7)	(since C++23)
template<class F> constexprauto transform_error( F&& f)const&&;	(8)	(since C++23)

If*this contains an unexpected value, invokesf with the unexpected value of*this as the argument and returns astd::expected object that contains an unexpected value, which is initialized with the result off. Otherwise, returns astd::expected object that represents an expected value.

1-4) The expected value is initialized with the expected valueval of*this.

Given typeG as:

1,2)std::remove_cv_t<std::invoke_result_t<F, decltype(error())>>

3,4)std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(error()))>>

5,6)std::remove_cv_t<std::invoke_result_t<F, decltype(error())>>

7,8)std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(error()))>>

If any of the following conditions is satisfied, the program is ill-formed:

G is not a valid template argument forstd::unexpected.
The following corresponding declaration is ill-formed:

1,2)G g(std::invoke(std::forward<F>(f), error()));

3,4)G g(std::invoke(std::forward<F>(f), std::move(error()));

5,6)G g(std::invoke(std::forward<F>(f), error()));

7,8)G g(std::invoke(std::forward<F>(f), std::move(error()));

1,2) This overload participates in overload resolution only ifstd::is_constructible_v<T, decltype((val))> istrue.

3,4) This overload participates in overload resolution only ifstd::is_constructible_v<T, decltype(std::move(val))> istrue.

[edit]Parameters

a suitable function orCallable object whose call signature returns a non-reference type

[edit]Return value

Given expressionexpr as:

1,2)std::invoke(std::forward<F>(f), error())

3,4)std::invoke(std::forward<F>(f), std::move(error()))

5,6)std::invoke(std::forward<F>(f), error())

7,8)std::invoke(std::forward<F>(f), std::move(error()))

The return values are defined as follows:

Overload	Value of`has_value()`
Overload	true	false
(1,2)	std::expected<T, G>(std::in_place, `val`)	std::expected<T, G> (std::unexpect, expr)
(3,4)	std::expected<T, G>(std::in_place, std::move(`val`))
(5,6)	std::expected<T, G>()
(7,8)	std::expected<T, G>()

[edit]Example

This section is incomplete
Reason: no example

[edit]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 byvalue()^[1]	changed to**this
LWG 3973	C++23	the expected value was obtained by**this^[2]	changed to`val`

↑value() requiresE to be copy constructible (seeLWG issue 3843), whereoperator* does not.
↑**this can triggerargument-dependent lookup.

[edit]See also

or_else	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)[edit]
transform	returns an`expected` containing the transformed expected value if it exists; otherwise, returns the`expected` itself (public member function)[edit]