23 General utilities library [utilities]

23.6 Optional objects [optional]

23.6.3 Class template optional [optional.optional]

23.6.3.1 Constructors [optional.ctor]

constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;

1
#

Postconditions: *this does not contain a value.

2
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Remarks: No contained value is initialized. For every object type T these constructors shall be constexpr constructors.

constexpr optional(const optional& rhs);

3
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Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.

4
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Postconditions: bool(rhs) == bool(*this).

5
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Throws: Any exception thrown by the selected constructor of T.

6
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Remarks: This constructor shall be defined as deleted unless is_­copy_­constructible_­v<T> is true. If is_­trivially_­copy_­constructible_­v<T> is true, this constructor shall be a constexpr constructor.

constexpr optional(optional&& rhs) noexcept(see below);

7
#

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs). bool(rhs) is unchanged.

8
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Postconditions: bool(rhs) == bool(*this).

9
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Throws: Any exception thrown by the selected constructor of T.

10
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Remarks: The expression inside noexcept is equivalent to is_­nothrow_­move_­constructible_­v<T>. This constructor shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true. If is_­trivially_­move_­constructible_­v<T> is true, this constructor shall be a constexpr constructor.

template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);

11
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Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std​::​forward<Args>(args)....

12
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Postconditions: *this contains a value.

13
#

Throws: Any exception thrown by the selected constructor of T.

14
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Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_­constructible_­v<T, Args...> is true.

template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

15
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Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std​::​forward<Args>(args)....

16
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Postconditions: *this contains a value.

17
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Throws: Any exception thrown by the selected constructor of T.

18
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Remarks: This constructor shall not participate in overload resolution unless is_­constructible_­v<T, initializer_­list<U>&, Args&&...> is true. If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.

19
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[ Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution.  — end note ]

template <class U = T> EXPLICIT constexpr optional(U&& v);

20
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Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​forward<U>(v).

21
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Postconditions: *this contains a value.

22
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Throws: Any exception thrown by the selected constructor of T.

23
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Remarks: If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_­constructible_­v<T, U&&> is true, is_­same_­v<decay_­t<U>, in_­place_­t> is false, and is_­same_­v<optional<T>, decay_­t<U>> is false. The constructor is explicit if and only if is_­convertible_­v<U&&, T> is false.

template <class U> EXPLICIT optional(const optional<U>& rhs);

24
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Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.

25
#

Postconditions: bool(rhs) == bool(*this).

26
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Throws: Any exception thrown by the selected constructor of T.

27
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Remarks: This constructor shall not participate in overload resolution unless

  • (27.1)

    is_­constructible_­v<T, const U&> is true,

  • (27.2)

    is_­constructible_­v<T, optional<U>&> is false,

  • (27.3)

    is_­constructible_­v<T, optional<U>&&> is false,

  • (27.4)

    is_­constructible_­v<T, const optional<U>&> is false,

  • (27.5)

    is_­constructible_­v<T, const optional<U>&&> is false,

  • (27.6)

    is_­convertible_­v<optional<U>&, T> is false,

  • (27.7)

    is_­convertible_­v<optional<U>&&, T> is false,

  • (27.8)

    is_­convertible_­v<const optional<U>&, T> is false, and

  • (27.9)

    is_­convertible_­v<const optional<U>&&, T> is false.

The constructor is explicit if and only if is_­convertible_­v<const U&, T> is false.

template <class U> EXPLICIT optional(optional<U>&& rhs);

28
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Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs). bool(rhs) is unchanged.

29
#

Postconditions: bool(rhs) == bool(*this).

30
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Throws: Any exception thrown by the selected constructor of T.

31
#

Remarks: This constructor shall not participate in overload resolution unless

  • (31.1)

    is_­constructible_­v<T, U&&> is true,

  • (31.2)

    is_­constructible_­v<T, optional<U>&> is false,

  • (31.3)

    is_­constructible_­v<T, optional<U>&&> is false,

  • (31.4)

    is_­constructible_­v<T, const optional<U>&> is false,

  • (31.5)

    is_­constructible_­v<T, const optional<U>&&> is false,

  • (31.6)

    is_­convertible_­v<optional<U>&, T> is false,

  • (31.7)

    is_­convertible_­v<optional<U>&&, T> is false,

  • (31.8)

    is_­convertible_­v<const optional<U>&, T> is false, and

  • (31.9)

    is_­convertible_­v<const optional<U>&&, T> is false.

The constructor is explicit if and only if is_­convertible_­v<U&&, T> is false.