22 General utilities library [utilities]
namespace std {
template<class T>
class optional<T&> {
public:
using value_type = T;
using iterator = implementation-defined;
public:
constexpr optional() noexcept = default;
constexpr optional(nullopt_t) noexcept : optional() {}
constexpr optional(const optional& rhs) noexcept = default;
template<class Arg>
constexpr explicit optional(in_place_t, Arg&& arg);
template<class U>
constexpr explicit(see below) optional(U&& u) noexcept(see below);
template<class U>
constexpr explicit(see below) optional(optional<U>& rhs) noexcept(see below);
template<class U>
constexpr explicit(see below) optional(const optional<U>& rhs) noexcept(see below);
template<class U>
constexpr explicit(see below) optional(optional<U>&& rhs) noexcept(see below);
template<class U>
constexpr explicit(see below) optional(const optional<U>&& rhs) noexcept(see below);
constexpr ~optional() = default;
constexpr optional& operator=(nullopt_t) noexcept;
constexpr optional& operator=(const optional& rhs) noexcept = default;
template<class U> constexpr T& emplace(U&& u) noexcept(see below);
constexpr void swap(optional& rhs) noexcept;
constexpr iterator begin() const noexcept;
constexpr iterator end() const noexcept;
constexpr T* operator->() const noexcept;
constexpr T& operator*() const noexcept;
constexpr explicit operator bool() const noexcept;
constexpr bool has_value() const noexcept;
constexpr T& value() const;
template<class U = remove_cv_t<T>>
constexpr remove_cv_t<T> value_or(U&& u) const;
template<class F> constexpr auto and_then(F&& f) const;
template<class F> constexpr optional<invoke_result_t<F, T&>> transform(F&& f) const;
template<class F> constexpr optional or_else(F&& f) const;
constexpr void reset() noexcept;
private:
T* val = nullptr;
template<class U>
constexpr void convert-ref-init-val(U&& u);
};
}
When an
optional<T&> contains a value,
the
contained value
is a reference to
*val.