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

From cppreference.com
< cpp‎ | utility‎ | expected
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::expected
 
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.

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