#include <functional>#include <iostream>#include <set> struct S{int x;    S(int i): x{i}{std::cout<<"S{"<< i<<"} ";}bool operator<(const R& s)const{return x< s.x;}}; struct R{int x;    R(int i): x{i}{std::cout<<"R{"<< i<<"} ";}bool operator<(const R& r)const{return x< r.x;}}; bool operator<(const R& r,int i){return r.x< i;}bool operator<(int i,const R& r){return i< r.x;} int main(){std::set<int> t{3,1,4,1,5};std::cout<< t.count(1)<<", "<< t.count(2)<<".\n"; std::set<S> s{3,1,4,1,5};std::cout<<": "<< s.count(1)<<", "<< s.count(2)<<".\n";// Two temporary objects S{1} and S{2} were created.// Comparison function object is defaulted std::less<S>,// which is not transparent (has no “is_transparent” nested type). std::set<R,std::less<>> r{3,1,4,1,5};std::cout<<": "<< r.count(1)<<", "<< r.count(2)<<".\n";// C++14 heterogeneous lookup; temporary objects were not created.// Comparator std::less<void> has predefined “is_transparent”.}

1, 0.S{3} S{1} S{4} S{1} S{5} : S{1} 1, S{2} 0.R{3} R{1} R{4} R{1} R{5} : 1, 0.