std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::swap - cppreference.com

From cppreference.com

void swap( flat_map& other ) noexcept;		(since C++23)

Exchanges the contents of the container adaptor with those of other. Effectively calls

ranges::swap(compare, other.compare);
ranges::swap(c.keys, other.c.keys);
ranges::swap(c.values, other.c.values);

other

container adaptor to exchange the contents with

Same as underlying container (typically constant).