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std::expected::or_else(3) C++ Standard Libary std::expected::or_else(3) NAME std::expected::or_else - std::expected::or_else Synopsis template< class F > (1) (since C++23) constexpr auto or_else( F&& f ) &; template< class F > (2) (since C++23) constexpr auto or_else( F&& f ) const&; template< class F > (3) (since C++23) constexpr auto or_else( F&& f ) &&; template< class F > (4) (since C++23) constexpr auto or_else( F&& f ) const&&; If *this contains an unexpected value, invokes f with the argument error() and returns its result; otherwise, returns a std::expected object that contains a copy of the contained expected value (obtained from operator*). 1,2) Given type G as std::remove_cvref_t<std::invoke_result_t<F, decltype(error())>>. If G is not a specialization of std::expected, or std::is_same_v<G::value_type, T> is false, the program is ill-formed. The effect is equivalent to if (has_value()) { if constexpr (std::is_void_v<T>) return G(); else return G(std::in_place, **this); } else return std::invoke(std::forward<F>(f), error()); These overloads participate in overload resolution only if std::is_void_v<T> or std::is_constructible_v<T, decltype(**this)> is true. 3,4) Given type G as std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(error()))>>. If G is not a specialization of std::expected, or std::is_same_v<G::value_type, T> is false, the program is ill-formed. The effect is equivalent to if (has_value()) { if constexpr (std::is_void_v<T>) return G(); else return G(std::in_place, std::move(**this)); } else return std::invoke(std::forward<F>(f), std::move(error())); These overloads participate in overload resolution only if std::is_void_v<T> or std::is_constructible_v<T, decltype(std::move(**this))> is true. Parameters f - a suitable function or Callable object that returns a std::ex- pected Return value The result of f, or a std::expected object that contains a copy of the expected value, as described above. Notes Feature-test macro Value Std Feature __cpp_lib_expected 202211L (C++23) Monadic functions for std::ex- pected 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 or_else 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::or_else(3)
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