[string.special]
27 Strings library [strings]
27.4 String classes [string.classes]
27.4.4 Non-member functions [string.nonmembers]
27.4.4.3 swap [string.special]
template<class charT, class traits, class Allocator>
constexpr void
swap(basic_string<charT, traits, Allocator>& lhs,
basic_string<charT, traits, Allocator>& rhs)
noexcept(noexcept(lhs.swap(rhs)));