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Showing content from https://timsong-cpp.github.io/cppwp/n4659/optional.swap below:

[optional.swap]

23 General utilities library [utilities] 23.6 Optional objects [optional] 23.6.3 Class template optional [optional.optional] 23.6.3.4 Swap [optional.swap]

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

Requires: Lvalues of type T shall be swappable and is_move_constructible_v<T> is true.

Effects: See Table 39.

Table

—

optional::swap(optional&)

effects

*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 39.

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.

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