#include <cassert>#include <iostream>#include <map> struct LightKey{int o;};struct HeavyKey{int o[1000];}; // The container must use std::less<> (or other transparent Comparator) to// access overloads (3,4). This includes standard overloads, such as// comparison between std::string and std::string_view.bool operator<(const HeavyKey& x,const LightKey& y){return x.o[0]< y.o;}bool operator<(const LightKey& x,const HeavyKey& y){return x.o< y.o[0];}bool operator<(const HeavyKey& x,const HeavyKey& y){return x.o[0]< y.o[0];} int main(){std::map<int,char> map{{1,'a'},{2,'b'}};assert(map.at(1)=='a');assert(map.at(2)=='b');try{        map.at(13);}catch(conststd::out_of_range& ex){std::cout<<"1) out_of_range::what(): "<< ex.what()<<'\n';} #ifdef __cpp_lib_associative_heterogeneous_insertion// Transparent comparison demo.std::map<HeavyKey,char,std::less<>> map2{{{1},'a'},{{2},'b'}};assert(map2.at(LightKey{1})=='a');assert(map2.at(LightKey{2})=='b');try{        map2.at(LightKey{13});}catch(conststd::out_of_range& ex){std::cout<<"2) out_of_range::what(): "<< ex.what()<<'\n';}#endif}

1) out_of_range::what(): map::at:  key not found2) out_of_range::what(): map::at:  key not found