19 General utilities library [utilities]

19.6 Optional objects [optional]

19.6.3 Class template optional [optional.optional]

19.6.3.1 Constructors [optional.ctor]

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constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;
1
#
Ensures: *this does not contain a value.
2
#
Remarks: No contained value is initialized.
For every object type T these constructors shall be constexpr constructors.
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constexpr optional(const optional& rhs);
3
#
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
#
Ensures: bool(rhs) == bool(*this).
5
#
Throws: Any exception thrown by the selected constructor of T.
6
#
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.
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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
#
Ensures: bool(rhs) == bool(*this).
9
#
Throws: Any exception thrown by the selected constructor of T.
10
#
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.
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template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
11
#
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std​::​forward<Args>(args)....
12
#
Ensures: *this contains a value.
13
#
Throws: Any exception thrown by the selected constructor of T.
14
#
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.
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template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
15
#
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
#
Ensures: *this contains a value.
17
#
Throws: Any exception thrown by the selected constructor of T.
18
#
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.
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template<class U = T> explicit(see below) constexpr optional(U&& v);
19
#
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​forward<U>(v).
20
#
Ensures: *this contains a value.
21
#
Throws: Any exception thrown by the selected constructor of T.
22
#
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<remove_­cvref_­t<U>, in_­place_­t> is false, and is_­same_­v<remove_­cvref_­t<U>, optional> is false.
The expression inside explicit is equivalent to: 
!is_convertible_v<U, T>
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template<class U> explicit(see below) optional(const optional<U>& rhs);
23
#
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.
24
#
Ensures: bool(rhs) == bool(*this).
25
#
Throws: Any exception thrown by the selected constructor of T.
26
#
Remarks: This constructor shall not participate in overload resolution unless
  • (26.1)
    is_­constructible_­v<T, const U&> is true,
  • (26.2)
    is_­constructible_­v<T, optional<U>&> is false,
  • (26.3)
    is_­constructible_­v<T, optional<U>&&> is false,
  • (26.4)
    is_­constructible_­v<T, const optional<U>&> is false,
  • (26.5)
    is_­constructible_­v<T, const optional<U>&&> is false,
  • (26.6)
    is_­convertible_­v<optional<U>&, T> is false,
  • (26.7)
    is_­convertible_­v<optional<U>&&, T> is false,
  • (26.8)
    is_­convertible_­v<const optional<U>&, T> is false, and
  • (26.9)
    is_­convertible_­v<const optional<U>&&, T> is false.
The expression inside explicit is equivalent to: 
!is_convertible_v<const U&, T>
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template<class U> explicit(see below) optional(optional<U>&& rhs);
27
#
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.
28
#
Ensures: bool(rhs) == bool(*this).
29
#
Throws: Any exception thrown by the selected constructor of T.
30
#
Remarks: This constructor shall not participate in overload resolution unless
  • (30.1)
    is_­constructible_­v<T, U&&> is true,
  • (30.2)
    is_­constructible_­v<T, optional<U>&> is false,
  • (30.3)
    is_­constructible_­v<T, optional<U>&&> is false,
  • (30.4)
    is_­constructible_­v<T, const optional<U>&> is false,
  • (30.5)
    is_­constructible_­v<T, const optional<U>&&> is false,
  • (30.6)
    is_­convertible_­v<optional<U>&, T> is false,
  • (30.7)
    is_­convertible_­v<optional<U>&&, T> is false,
  • (30.8)
    is_­convertible_­v<const optional<U>&, T> is false, and
  • (30.9)
    is_­convertible_­v<const optional<U>&&, T> is false.
The expression inside explicit is equivalent to: 
!is_convertible_v<U, T>