std::optional<T>::swap (3) - Linux Man Pages
void swap( optional& other ) noexcept(/* see below */); (since C++17)
Swaps the contents with those of other.
* If neither *this nor other contain a value, the function has no effect.
* If only one of *this and other contains a value (let's call this object in and the other un), the contained value of un is direct-initialized from std::move(*in), followed by destruction of the contained value of in as if by in->T::~T(). After this call, in does not contain a value; un contains a value.
* If both *this and other contain values, the contained values are exchanged by calling using std::swap; swap(**this, *other). T lvalues must satisfy Swappable.
other - the optional object to exchange the contents with
In the case of thrown exception, the states of the contained values of *this and other are determined by the exception safety guarantees of swap of type T or T's move constructor, whichever is called. For both *this and other, if the object contained a value, it is left containing a value, and the other way round.
std::swap(std::optional) specializes the std::swap algorithm