#include <iostream>#include <map> struct LightKey{int x;}; struct FatKey{int x;int data[1000];// a heavy blob}; // As detailed above, the container must use std::less<> (or other transparent// Comparator) to access these overloads. This includes standard overloads,// such as comparison between std::string and std::string_view.bool operator<(const FatKey& fk,const LightKey& lk){return fk.x< lk.x;}bool operator<(const LightKey& lk,const FatKey& fk){return lk.x< fk.x;}bool operator<(const FatKey& fk1,const FatKey& fk2){return fk1.x< fk2.x;} int main(){// Simple comparison demo.std::multimap<int,char> example{{1,'a'},{2,'b'}}; if(auto search= example.find(2); search!= example.end())std::cout<<"Found "<< search->first<<' '<< search->second<<'\n';elsestd::cout<<"Not found\n"; // Transparent comparison demo.std::multimap<FatKey,char,std::less<>> example2{{{1,{}},'a'},{{2,{}},'b'}};     LightKey lk={2};if(auto search= example2.find(lk); search!= example2.end())std::cout<<"Found "<< search->first.x<<' '<< search->second<<'\n';elsestd::cout<<"Not found\n"; // Obtaining const iterators.// Compiler decides whether to return iterator of (non) const type by way of// accessing map; to prevent intentional modification, one of the simplest// options is to access the map via a constant reference.constauto& example2ref= example2;if(auto search= example2ref.find(lk); search!= example2.end()){std::cout<<"Found "<< search->first.x<<' '<< search->second<<'\n';//  search->second = 'c'; // error: assignment of member// 'std::pair<const FatKey, char>::second'// in read-only object}}

Found 2 bFound 2 bFound 2 b