#include <functional>#include <iostream>#include <set> template<class Os,class Co>Os& operator<<(Os& os,const Co& co){    os<<'{';for(constauto& i: co)        os<<' '<< i;return os<<" } ";} int main(){std::set<int> a1{3,1,3,2}, a2{5,4,5}; auto it1=std::next(a1.begin());auto it2=std::next(a2.begin()); constint& ref1=*(a1.begin());constint& ref2=*(a2.begin()); std::cout<< a1<< a2<<*it1<<' '<<*it2<<' '<< ref1<<' '<< ref2<<'\n';    a1.swap(a2);std::cout<< a1<< a2<<*it1<<' '<<*it2<<' '<< ref1<<' '<< ref2<<'\n'; // Note that every iterator referring to an element in one container before the swap// refers to the same element in the other container after the swap. Same is true// for references. struct Cmp:std::less<int>{int id{};        Cmp(int i): id{i}{}}; std::set<int, Cmp> s1{{2,2,1,1}, Cmp{6}}, s2{{4,4,3,3}, Cmp{9}}; std::cout<< s1<< s2<< s1.key_comp().id<<' '<< s2.key_comp().id<<'\n';    s1.swap(s2);std::cout<< s1<< s2<< s1.key_comp().id<<' '<< s2.key_comp().id<<'\n'; // So, comparator objects (Cmp) are also exchanged after the swap.}

{ 1 2 3 } { 4 5 } 2 5 1 4{ 4 5 } { 1 2 3 } 2 5 1 4{ 1 2 } { 3 4 } 6 9{ 3 4 } { 1 2 } 9 6