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; // see [optional.ref.iterators] public: // [optional.ref.ctor], constructors 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; // [optional.ref.assign], assignment 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); // [optional.ref.swap], swap constexpr void swap(optional& rhs) noexcept; // [optional.ref.iterators], iterator support constexpr iterator begin() const noexcept; constexpr iterator end() const noexcept; // [optional.ref.observe], observers 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; // freestanding-deleted template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const; // [optional.ref.monadic], monadic operations 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; // [optional.ref.mod], modifiers constexpr void reset() noexcept; private: T* val = nullptr; // exposition only // [optional.ref.expos], exposition only helper functions template<class U> constexpr void convert-ref-init-val(U&& u); // exposition only }; }
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An object of type optional<T&> contains a value if and only if val != nullptr is true.
When an optional<T&> contains a value, the contained value is a reference to *val.

22.5.4.2 Constructors [optional.ref.ctor]

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template<class Arg> constexpr explicit optional(in_place_t, Arg&& arg);
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Constraints:
  • (1.1)
    is_constructible_v<T&, Arg> is true, and
  • (1.2)
    reference_constructs_from_temporary_v<T&, Arg> is false.
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<Arg>(arg)).
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Postconditions: *this contains a value.
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template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);
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Constraints:
  • (4.1)
    is_same_v<remove_cvref_t<U>, optional> is false,
  • (4.2)
    is_same_v<remove_cvref_t<U>, in_place_t> is false, and
  • (4.3)
    is_constructible_v<T&, U> is true.
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).
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Postconditions: *this contains a value.
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U> is true.
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template<class U> constexpr explicit(!is_convertible_v<U&, T&>) optional(optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, U&>);
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Constraints:
  • (8.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (8.2)
    is_same_v<T&, U> is false, and
  • (8.3)
    is_constructible_v<T&, U&> is true.
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*rhs);
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U&> is true.
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template<class U> constexpr explicit(!is_convertible_v<const U&, T&>) optional(const optional<U>& rhs) noexcept(is_nothrow_constructible_v<T&, const U&>);
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Constraints:
  • (11.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (11.2)
    is_same_v<T&, U> is false, and
  • (11.3)
    is_constructible_v<T&, const U&> is true.
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*rhs);
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, const U&> is true.
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template<class U> constexpr explicit(!is_convertible_v<U, T&>) optional(optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, U>);
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Constraints:
  • (14.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (14.2)
    is_same_v<T&, U> is false, and
  • (14.3)
    is_constructible_v<T&, U> is true.
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, U> is true.
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template<class U> constexpr explicit(!is_convertible_v<const U, T&>) optional(const optional<U>&& rhs) noexcept(is_nothrow_constructible_v<T&, const U>);
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Constraints:
  • (17.1)
    is_same_v<remove_cv_t<T>, optional<U>> is false,
  • (17.2)
    is_same_v<T&, U> is false, and
  • (17.3)
    is_constructible_v<T&, const U> is true.
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Effects: Equivalent to: if (rhs.has_value()) convert-ref-init-val(*std::move(rhs));
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Remarks: This constructor is defined as deleted if reference_constructs_from_temporary_v<T&, const U> is true.

22.5.4.3 Assignment [optional.ref.assign]

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constexpr optional& operator=(nullopt_t) noexcept;
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Effects: Assigns nullptr to val.
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Postconditions: *this does not contain a value.
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Returns: *this.
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template<class U> constexpr T& emplace(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);
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Constraints:
  • (4.1)
    is_constructible_v<T&, U> is true, and
  • (4.2)
    reference_constructs_from_temporary_v<T&, U> is false.
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).

22.5.4.4 Swap [optional.ref.swap]

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constexpr void swap(optional& rhs) noexcept;
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Effects: Equivalent to: swap(val, rhs.val).

22.5.4.5 Iterator support [optional.ref.iterators]

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using iterator = implementation-defined;
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This type models contiguous_iterator ([iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]), and meets the requirements for constexpr iterators ([iterator.requirements.general]), with value type remove_cv_t<T>.
The reference type is T& for iterator.
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All requirements on container iterators ([container.reqmts]) apply to optional​::​iterator.
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constexpr iterator begin() const noexcept;
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Returns: If has_value() is true, an iterator referring to *val.
Otherwise, a past-the-end iterator value.
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constexpr iterator end() const noexcept;
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Returns: begin() + has_value().

22.5.4.6 Observers [optional.ref.observe]

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constexpr T* operator->() const noexcept;
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Hardened preconditions: has_value() is true.
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Returns: val.
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constexpr T& operator*() const noexcept;
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Hardened preconditions: has_value() is true.
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Returns: *val.
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constexpr explicit operator bool() const noexcept;
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Returns: val != nullptr.
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constexpr bool has_value() const noexcept;
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Returns: val != nullptr.
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constexpr T& value() const;
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Effects: Equivalent to: return has_value() ? *val : throw bad_optional_access();
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template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;
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Let X be remove_cv_t<T>.
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Mandates: is_constructible_v<X, T&> && is_convertible_v<U, X> is true.
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Effects: Equivalent to: return has_value() ? *val : static_cast<X>(std::forward<U>(u));

22.5.4.7 Monadic operations [optional.ref.monadic]

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template<class F> constexpr auto and_then(F&& f) const;
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Let U be invoke_result_t<F, T&>.
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Mandates: remove_cvref_t<U> is a specialization of optional.
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Effects: Equivalent to: if (has_value()) { return invoke(std::forward<F>(f), *val); } else { return remove_cvref_t<U>(); }
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template<class F> constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;
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Let U be remove_cv_t<invoke_result_t<F, T&>>.
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Mandates: The declaration U u(invoke(std::forward<F>(f), *val)); is well-formed for some invented variable u.
[Note 1: 
There is no requirement that U is movable ([dcl.init.general]).
— end note]
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Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std​::​forward<F>(f), *val); otherwise, optional<U>().
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template<class F> constexpr optional or_else(F&& f) const;
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Constraints: F models invocable.
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Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.
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Effects: Equivalent to: if (has_value()) { return *val; } else { return std::forward<F>(f)(); }

22.5.4.8 Modifiers [optional.ref.mod]

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constexpr void reset() noexcept;
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Effects: Assigns nullptr to val.
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Postconditions: *this does not contain a value.

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

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template<class U> constexpr void convert-ref-init-val(U&& u); // exposition only
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Effects: Creates a variable r as if by T& r(std​::​forward<U>(u)); and then initializes val with addressof(r).