22 General utilities library [utilities]

22.8 Expected objects [expected]

22.8.6 Class template expected [expected.expected]

22.8.6.1 General [expected.object.general]

namespace std { template<class T, class E> class expected { public: using value_type = T; using error_type = E; using unexpected_type = unexpected<E>; template<class U> using rebind = expected<U, error_type>; // [expected.object.cons], constructors constexpr expected(); constexpr expected(const expected&); constexpr expected(expected&&) noexcept(see below); template<class U, class G> constexpr explicit(see below) expected(const expected<U, G>&); template<class U, class G> constexpr explicit(see below) expected(expected<U, G>&&); template<class U = remove_cv_t<T>> constexpr explicit(see below) expected(U&& v); template<class G> constexpr explicit(see below) expected(const unexpected<G>&); template<class G> constexpr explicit(see below) expected(unexpected<G>&&); template<class... Args> constexpr explicit expected(in_place_t, Args&&...); template<class U, class... Args> constexpr explicit expected(in_place_t, initializer_list<U>, Args&&...); template<class... Args> constexpr explicit expected(unexpect_t, Args&&...); template<class U, class... Args> constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...); // [expected.object.dtor], destructor constexpr ~expected(); // [expected.object.assign], assignment constexpr expected& operator=(const expected&); constexpr expected& operator=(expected&&) noexcept(see below); template<class U = remove_cv_t<T>> constexpr expected& operator=(U&&); template<class G> constexpr expected& operator=(const unexpected<G>&); template<class G> constexpr expected& operator=(unexpected<G>&&); template<class... Args> constexpr T& emplace(Args&&...) noexcept; template<class U, class... Args> constexpr T& emplace(initializer_list<U>, Args&&...) noexcept; // [expected.object.swap], swap constexpr void swap(expected&) noexcept(see below); friend constexpr void swap(expected& x, expected& y) noexcept(noexcept(x.swap(y))); // [expected.object.obs], observers constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept; constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept; constexpr const T&& operator*() const && noexcept; constexpr T&& operator*() && noexcept; constexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr const T& value() const &; // freestanding-deleted constexpr T& value() &; // freestanding-deleted constexpr const T&& value() const &&; // freestanding-deleted constexpr T&& value() &&; // freestanding-deleted constexpr const E& error() const & noexcept; constexpr E& error() & noexcept; constexpr const E&& error() const && noexcept; constexpr E&& error() && noexcept; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&; template<class G = E> constexpr E error_or(G&&) const &; template<class G = E> constexpr E error_or(G&&) &&; // [expected.object.monadic], monadic operations template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &; template<class F> constexpr auto and_then(F&& f) const &&; template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &; template<class F> constexpr auto or_else(F&& f) const &&; template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &; template<class F> constexpr auto transform(F&& f) const &&; template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &; template<class F> constexpr auto transform_error(F&& f) const &&; // [expected.object.eq], equality operators template<class T2, class E2> requires (!is_void_v<T2>) friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y); template<class T2> friend constexpr bool operator==(const expected&, const T2&); template<class E2> friend constexpr bool operator==(const expected&, const unexpected<E2>&); private: bool has_val; // exposition only union { T val; // exposition only E unex; // exposition only }; }; }
1
#
Any object of type expected<T, E> either contains a value of type T or a value of type E nested within ([intro.object]) it.
Member has_val indicates whether the expected<T, E> object contains an object of type T.
2
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A type T is a valid value type for expected, if remove_cv_t<T> is void or a complete non-array object type that is not in_place_t, unexpect_t, or a specialization of unexpected.
A program which instantiates class template expected<T, E> with an argument T that is not a valid value type for expected is ill-formed.
A program that instantiates the definition of the template expected<T, E> with a type for the E parameter that is not a valid template argument for unexpected is ill-formed.
3
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When T is not cv void, it shall meet the Cpp17Destructible requirements (Table 35).
E shall meet the Cpp17Destructible requirements.