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

 

C++

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Freestanding and hosted

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Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) 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) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (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 Date and time Function objects Formatting library (C++20)   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 and type 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) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

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

 

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.

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

5-8) f is invoked without any argument.

Given type U 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 for std::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)	If std::is_void_v<U> is false, returns std::expected<U, E>     (std::in_place, expr). Otherwise, returns std::expected<U, E>() (but still evaluates expr).	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()))

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()	changed to **this
LWG 3973	C++23	the expected value was obtained by **this	changed to val

See also

transform_error

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