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

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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

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bitset
hash (C++11)

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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)

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swap
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declval (C++11)
to_underlying (C++23)

forward (C++11)
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move (C++11)
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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)

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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

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

If*this represents an expected value, invokesf and returns astd::expected object that contains an expected value, which is initialized with its result (or value-initialized if the result type isvoid). Otherwise, returns astd::expected object that contains an unexpected value, which is initialized with the unexpected value of*this.

1-4)f is invoked with the expected valueval of*this as the argument.

5-8)f is invoked without any argument.

Given typeU as:

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

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

5-8)std::remove_cv_t<std::invoke_result_t<F>>

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

U is not a valid value type forstd::expected.
std::is_void_v<U> isfalse, 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 ifstd::is_constructible_v<E, decltype(error())> istrue.

3,4) These overloads participate in overload resolution only ifstd::is_constructible_v<E, decltype(std::move(error()))> istrue.

5,6) These overloads participate in overload resolution only ifstd::is_constructible_v<E, decltype(error())> istrue.

7,8) These overloads participate in overload resolution only ifstd::is_constructible_v<E, decltype(std::move(error()))> 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), 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()`
Overload	true	false
(1,2)	Ifstd::is_void_v<U> isfalse, returnsstd::expected<U, E> (std::in_place, expr). Otherwise, returnsstd::expected<U, E>() (but still evaluatesexpr).	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()))

[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

transform_error

returns theexpected itself if it contains an expected value; otherwise, returns anexpected containing the transformed unexpected value
(public member function)[edit]

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