#include <iostream>#include <string>#include <utility>#include <map> // print out a std::pairtemplate<class Os,class U,class V>Os& operator<<(Os& os,conststd::pair<U, V>& p){return os<< p.first<<':'<< p.second;} // print out a containertemplate<class Os,class Co>Os& operator<<(Os& os,const Co& co){    os<<'{';for(constauto& i: co)        os<<' '<< i;return os<<" }\n";} int main(){std::map<std::string,std::string>        m1{{"γ","gamma"},{"β","beta"},{"α","alpha"},{"γ","gamma"}},        m2{{"ε","epsilon"},{"δ","delta"},{"ε","epsilon"}}; constauto& ref=*(m1.begin());constauto iter=std::next(m1.cbegin()); std::cout<<"──────── before swap ────────\n"<<"m1: "<< m1<<"m2: "<< m2<<"ref: "<< ref<<"\niter: "<<*iter<<'\n';     m1.swap(m2); std::cout<<"──────── after swap ────────\n"<<"m1: "<< m1<<"m2: "<< m2<<"ref: "<< ref<<"\niter: "<<*iter<<'\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.}

──────── before swap ────────m1: { α:alpha β:beta γ:gamma }m2: { δ:delta ε:epsilon }ref: α:alphaiter: β:beta──────── after swap ────────m1: { δ:delta ε:epsilon }m2: { α:alpha β:beta γ:gamma }ref: α:alphaiter: β:beta