[optional.optional.ref] (original) (raw)

22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.4 Partial specialization of optional for reference types [optional.optional.ref]

22.5.4.1 General [optional.optional.ref.general]

22.5.4.2 Constructors [optional.ref.ctor]

22.5.4.3 Assignment [optional.ref.assign]

22.5.4.4 Swap [optional.ref.swap]

22.5.4.5 Iterator support [optional.ref.iterators]

22.5.4.6 Observers [optional.ref.observe]

22.5.4.7 Monadic operations [optional.ref.monadic]

22.5.4.8 Modifiers [optional.ref.mod]

22.5.4.9 Exposition only helper functions [optional.ref.expos]

22.5.4.1 General [optional.optional.ref.general]

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); };}

An object of type optional<T&> contains a valueif and only if val != nullptr is true.

When an optional<T&> contains a value, the contained valueis a reference to *val.

22.5.4.2 Constructors [optional.ref.ctor]

template<class Arg> constexpr explicit optional(in_place_t, Arg&& arg);

Constraints:

Effects: Equivalent to: convert-ref-init-val(std​::​forward<Arg>(arg)).

Postconditions: *this contains a value.

template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);

Constraints:

Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).

Postconditions: *this contains a value.

Remarks: This constructor is defined as deleted ifreference_constructs_from_temporary_v<T&, U> is true.

template<class U> constexpr explicit(!is_convertible_v<U&, T&>) optional(optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, U&>);

Constraints:

Effects: Equivalent to:if (rhs.has_value()) convert-ref-init-val(*rhs);

Remarks: This constructor is defined as deleted ifreference_constructs_from_temporary_v<T&, U&> is true.

template<class U> constexpr explicit(!is_convertible_v<const U&, T&>) optional(const optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, const U&>);

Constraints:

Effects: Equivalent to:if (rhs.has_value()) convert-ref-init-val(*rhs);

Remarks: This constructor is defined as deleted ifreference_constructs_from_temporary_v<T&, const U&> is true.

template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, U>);

Constraints:

Effects: Equivalent to:if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));

Remarks: This constructor is defined as deleted ifreference_constructs_from_temporary_v<T&, U> is true.

template<class U> constexpr explicit(!is_convertible_v<const U, T&>) optional(const optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, const U>);

Constraints:

Effects: Equivalent to:if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));

Remarks: This constructor is defined as deleted ifreference_constructs_from_temporary_v<T&, const U> is true.

22.5.4.3 Assignment [optional.ref.assign]

constexpr optional& operator=(nullopt_t) noexcept;

Effects: Assigns nullptr to val.

Postconditions: *this does not contain a value.

template<class U> constexpr T& emplace(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);

Constraints:

Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).

22.5.4.4 Swap [optional.ref.swap]

constexpr void swap(optional& rhs) noexcept;

Effects: Equivalent to: swap(val, rhs.val).

22.5.4.5 Iterator support [optional.ref.iterators]

using iterator = _implementation-defined_;

The reference type is T& for iterator.

constexpr iterator begin() const noexcept;

Returns: If has_value() is true, an iterator referring to *val.

Otherwise, a past-the-end iterator value.

constexpr iterator end() const noexcept;

Returns: begin() + has_value().

22.5.4.6 Observers [optional.ref.observe]

constexpr T* operator->() const noexcept;

Hardened preconditions: has_value() is true.

constexpr T& operator*() const noexcept;

Hardened preconditions: has_value() is true.

constexpr explicit operator bool() const noexcept;

constexpr bool has_value() const noexcept;

constexpr T& value() const;

Effects: Equivalent to:return has_value() ? *val : throw bad_optional_access();

template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;

Mandates: is_constructible_v<X, T&> && is_convertible_v<U, X> is true.

Effects: Equivalent to:return has_value() ? *val : static_cast<X>(std::forward<U>(u));

22.5.4.7 Monadic operations [optional.ref.monadic]

template<class F> constexpr auto and_then(F&& f) const;

Let U be invoke_result_t<F, T&>.

Mandates: remove_cvref_t<U> is a specialization of optional.

Effects: Equivalent to:if (has_value()) { return invoke(std::forward<F>(f), *val);} else { return remove_cvref_t<U>();}

template<class F> constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;

Let U be remove_cv_t<invoke_result_t<F, T&>>.

Mandates: The declarationU u(invoke(std::forward<F>(f), *val));is well-formed for some invented variable u.

Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized withinvoke(std​::​forward<F>(f), *val); otherwise, optional<U>().

template<class F> constexpr optional or_else(F&& f) const;

Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.

Effects: Equivalent to:if (has_value()) { return *val;} else { return std::forward<F>(f)();}

22.5.4.8 Modifiers [optional.ref.mod]

constexpr void reset() noexcept;

Effects: Assigns nullptr to val.

Postconditions: *this does not contain a value.

22.5.4.9 Exposition only helper functions [optional.ref.expos]

template<class U> constexpr void _convert-ref-init-val_(U&& u); // _exposition only_

Effects: Creates a variable r as if by T& r(std​::​forward<U>(u));and then initializes val with addressof(r).