20 General utilities library [utilities]

20.6 Optional objects [optional]

20.6.3 Class template optional [optional.optional]

20.6.3.2 Constructors [optional.ctor]

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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, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.
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Postconditions: bool(rhs) == bool(*this).
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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, 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.
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Postconditions: bool(rhs) == bool(*this).
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Throws: Any exception thrown by the selected constructor of T.
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Remarks: The expression inside noexcept 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: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments 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: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments 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: 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.
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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).
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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> explicit(see below) optional(const optional<U>& rhs);
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Constraints:
  • (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.
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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.
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Postconditions: bool(rhs) == bool(*this).
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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> explicit(see below) optional(optional<U>&& rhs);
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Constraints:
  • (32.1)
    is_­constructible_­v<T, U> is true,
  • (32.2)
    is_­constructible_­v<T, optional<U>&> is false,
  • (32.3)
    is_­constructible_­v<T, optional<U>&&> is false,
  • (32.4)
    is_­constructible_­v<T, const optional<U>&> is false,
  • (32.5)
    is_­constructible_­v<T, const optional<U>&&> is false,
  • (32.6)
    is_­convertible_­v<optional<U>&, T> is false,
  • (32.7)
    is_­convertible_­v<optional<U>&&, T> is false,
  • (32.8)
    is_­convertible_­v<const optional<U>&, T> is false, and
  • (32.9)
    is_­convertible_­v<const optional<U>&&, T> is false.
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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.
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Postconditions: bool(rhs) == bool(*this).
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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>