#include <algorithm>#include <functional>#include <iostream>#include <ranges>#include <string> struct Pair{int n;std::string s;friendstd::ostream& operator<<(std::ostream& os,const Pair& p){return os<<'{'<< p.n<<", "<< p.s<<'}';}}; // A range-printer that can print projected (modified) elements of a range.template<std::ranges::input_range R,typename Projection= std::identity>//<- Notice the default projectionvoid print(std::string_viewconst rem, R&& range, Projection projection={}){std::cout<< rem<<'{';    std::ranges::for_each(        range,[O=0](constauto& o) mutable{std::cout<<(O++?", ":"")<< o;},        projection);std::cout<<"}\n";} int main(){constauto v={Pair{1,"one"},{2,"two"},{3,"three"}};     print("Print using std::identity as a projection: ", v);    print("Project the Pair::n: ", v,&Pair::n);    print("Project the Pair::s: ", v,&Pair::s);    print("Print using custom closure as a projection: ", v,[](Pairconst& p){returnstd::to_string(p.n)+':'+ p.s;});}

Print using std::identity as a projection: {{1, one}, {2, two}, {3, three}}Project the Pair::n: {1, 2, 3}Project the Pair::s: {one, two, three}Print using custom closure as a projection: {1:one, 2:two, 3:three}