22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.3 Class template optional [optional.optional]

22.5.3.2 Constructors [optional.ctor]

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The exposition-only variable template converts-from-any-cvref is used by some constructors for optional.
template<class T, class W> constexpr bool converts-from-any-cvref = // exposition only disjunction_v<is_constructible<T, W&>, is_convertible<W&, T>, is_constructible<T, W>, is_convertible<W, T>, is_constructible<T, const W&>, is_convertible<const W&, T>, is_constructible<T, const W>, is_convertible<const W, T>>;
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constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;
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Postconditions: *this does not contain a value.
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Remarks: No contained value is initialized.
For every object type T these constructors are constexpr constructors ([dcl.constexpr]).
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constexpr optional(const optional& rhs);
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Effects: If rhs contains a value, direct-non-list-initializes the contained value with *rhs.
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Postconditions: rhs.has_value() == this->has_value().
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: This constructor is defined as deleted unless is_copy_constructible_v<T> is true.
If is_trivially_copy_constructible_v<T> is true, this constructor is trivial.
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constexpr optional(optional&& rhs) noexcept(see below);
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Constraints: is_move_constructible_v<T> is true.
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Effects: If rhs contains a value, direct-non-list-initializes the contained value with std​::​move(*rhs).
rhs.has_value() is unchanged.
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Postconditions: rhs.has_value() == this->has_value().
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<T>.
If is_trivially_move_constructible_v<T> is true, this constructor is trivial.
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template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
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Constraints: is_constructible_v<T, Args...> is true.
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Effects: Direct-non-list-initializes the contained value with std​::​forward<Args>(args)....
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Postconditions: *this contains a value.
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.
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template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
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Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true.
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Effects: Direct-non-list-initializes the contained value with il, std​::​forward<Args>(args)....
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Postconditions: *this contains a value.
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.
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template<class U = T> constexpr explicit(see below) optional(U&& v);
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Constraints:
  • (23.1)
    is_constructible_v<T, U> is true,
  • (23.2)
    is_same_v<remove_cvref_t<U>, in_place_t> is false,
  • (23.3)
    is_same_v<remove_cvref_t<U>, optional> is false, and
  • (23.4)
    if T is cv bool, remove_cvref_t<U> is not a specialization of optional.
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Effects: Direct-non-list-initializes the contained value with std​::​forward<U>(v).
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Postconditions: *this contains a value.
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.
The expression inside explicit is equivalent to: !is_convertible_v<U, T>
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template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);
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Constraints:
  • (28.1)
    is_constructible_v<T, const U&> is true, and
  • (28.2)
    if T is not cv bool, converts-from-any-cvref<T, optional<U>> is false.
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Effects: If rhs contains a value, direct-non-list-initializes the contained value with *rhs.
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Postconditions: rhs.has_value() == this->has_value().
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: The expression inside explicit is equivalent to: !is_convertible_v<const U&, T>
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template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);
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Constraints:
  • (33.1)
    is_constructible_v<T, U> is true, and
  • (33.2)
    if T is not cv bool, converts-from-any-cvref<T, optional<U>> is false.
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Effects: If rhs contains a value, direct-non-list-initializes the contained value with std​::​move(*rhs).
rhs.has_value() is unchanged.
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Postconditions: rhs.has_value() == this->has_value().
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: The expression inside explicit is equivalent to: !is_convertible_v<U, T>