#include <cstddef>#include <functional>#include <iostream>#include <source_location>#include <string>#include <string_view>#include <unordered_set> usingnamespace std::literals; namespace logger{bool enabled{false};} inlinevoid who(conststd::source_location sloc= std::source_location::current()){if(logger::enabled)std::cout<< sloc.function_name()<<'\n';} struct string_hash// C++20's transparent hashing{using hash_type=std::hash<std::string_view>;using is_transparent=void; std::size_t operator()(constchar* str)const{        who();return hash_type{}(str);}std::size_t operator()(std::string_view str)const{        who();return hash_type{}(str);}std::size_t operator()(conststd::string& str)const{        who();return hash_type{}(str);}}; int main(){std::unordered_set<int> example{1,2,-10}; std::cout<<"Simple comparison demo:\n"<<std::boolalpha;if(auto search= example.find(2); search!= example.end())std::cout<<"Found "<<*search<<'\n';elsestd::cout<<"Not found\n"; std::unordered_set<std::string, string_hash,std::equal_to<>> set{"one"s,"two"s};     logger::enabled=true;std::cout<<"Heterogeneous lookup for unordered containers (transparent hashing):\n"<<(set.find("one")!= set.end())<<'\n'<<(set.find("one"s)!= set.end())<<'\n'<<(set.find("one"sv)!= set.end())<<'\n';}

Simple comparison demo:Found 2Heterogeneous lookup for unordered containers (transparent hashing):std::size_t string_hash::operator()(const char*) consttruestd::size_t string_hash::operator()(const std::string&) consttruestd::size_t string_hash::operator()(std::string_view) consttrue