22 General utilities library [utilities]
22.8.7 Partial specialization of expected for void types [expected.void]
template<class T, class E> requires is_void_v<T>
class expected<T, E> {
public:
using value_type = T;
using error_type = E;
using unexpected_type = unexpected<E>;
template<class U>
using rebind = expected<U, error_type>;
constexpr expected() noexcept;
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 G>
constexpr explicit(see below) expected(const unexpected<G>&);
template<class G>
constexpr explicit(see below) expected(unexpected<G>&&);
constexpr explicit expected(in_place_t) noexcept;
template<class... Args>
constexpr explicit expected(unexpect_t, Args&&...);
template<class U, class... Args>
constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);
constexpr ~expected();
constexpr expected& operator=(const expected&);
constexpr expected& operator=(expected&&) noexcept(see below);
template<class G>
constexpr expected& operator=(const unexpected<G>&);
template<class G>
constexpr expected& operator=(unexpected<G>&&);
constexpr void emplace() noexcept;
constexpr void swap(expected&) noexcept(see below);
friend constexpr void swap(expected& x, expected& y) noexcept(noexcept(x.swap(y)));
constexpr explicit operator bool() const noexcept;
constexpr bool has_value() const noexcept;
constexpr void operator*() const noexcept;
constexpr void value() const &;
constexpr void value() &&;
constexpr const E& error() const & noexcept;
constexpr E& error() & noexcept;
constexpr const E&& error() const && noexcept;
constexpr E&& error() && noexcept;
template<class G = E> constexpr E error_or(G&&) const &;
template<class G = E> constexpr E error_or(G&&) &&;
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 &&;
template<class T2, class E2> requires is_void_v<T2>
friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
template<class E2>
friend constexpr bool operator==(const expected&, const unexpected<E2>&);
private:
bool has_val;
union {
E unex;
};
};
Any object of type
expected<T, E> either
represents a value of type
T, or
contains a value of type
E within its own storage
.Implementations are not permitted to use additional storage,
such as dynamic memory, to allocate the object of type
E.Member
has_val indicates whether the
expected<T, E> object
represents a value of type
T.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
.E shall meet the requirements of
Cpp17Destructible (Table
35)
.