constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;
Postconditions: *this does not contain a value.
Remarks: No contained value is initialized. For every object type T these constructors shall be constexpr constructors.
constexpr optional(const optional& rhs);
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.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
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);
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.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
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);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
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);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std::forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
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.
[ 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);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::forward<U>(v).
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
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);
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.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall not participate in overload resolution unless
is_constructible_v<T, const U&> is true,
is_constructible_v<T, optional<U>&> is false,
is_constructible_v<T, optional<U>&&> is false,
is_constructible_v<T, const optional<U>&> is false,
is_constructible_v<T, const optional<U>&&> is false,
is_convertible_v<optional<U>&, T> is false,
is_convertible_v<optional<U>&&, T> is false,
is_convertible_v<const optional<U>&, T> is false, and
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);
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.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall not participate in overload resolution unless
is_constructible_v<T, U&&> is true,
is_constructible_v<T, optional<U>&> is false,
is_constructible_v<T, optional<U>&&> is false,
is_constructible_v<T, const optional<U>&> is false,
is_constructible_v<T, const optional<U>&&> is false,
is_convertible_v<optional<U>&, T> is false,
is_convertible_v<optional<U>&&, T> is false,
is_convertible_v<const optional<U>&, T> is false, and
is_convertible_v<const optional<U>&&, T> is false.
The constructor is explicit if and only if is_convertible_v<U&&, T> is false.
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