20 General utilities library [utilities]

20.6 Optional objects [optional]

20.6.3 Class template optional [optional.optional]

20.6.3.4 Swap [optional.swap]

void swap(optional& rhs) noexcept(see below);

Mandates: is_move_constructible_v<T> is true.

Preconditions: Lvalues of type T are swappable.

Effects: See Table 46 .

Table 46: optional::swap(optional&) effects [tab:optional.swap]

	*this contains a value	*this does not contain a value
rhs contains a value	calls swap((this), *rhs)	initializes the contained value of this as if direct-non-list-initializing an object of type T with the expression std::move(rhs), followed by rhs.val->T::~T(); postcondition is that *this contains a value and rhs does not contain a value
rhs does not contain a value	initializes the contained value of rhs as if direct-non-list-initializing an object of type T with the expression std::move((this)), followed by val->T::~T(); postcondition is that *this does not contain a value and rhs contains a value	no effect

Throws: Any exceptions thrown by the operations in the relevant part of Table 46 .

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>

If any exception is thrown, the results of the expressions bool(*this) and bool(rhs) remain unchanged.

If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T.

If an exception is thrown during the call to T's move constructor, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.