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std::expected::transform(3) C++ Standard Libary std::expected::transform(3) 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 (ob- tained from operator*) is passed as an argument to f; otherwise f takes no argu- ment. 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)>> oth- erwise, 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, de- cltype(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 re- turns 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 retroac- tively 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 ex- pected value; transform_error otherwise, returns an expected containing the trans- formed unexpected value (public member function) Category: * Todo no example http://cppreference.com 2024.06.10 std::expected::transform(3)
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