#include <chrono>#include <functional>#include <iostream>#include <mutex>#include <string>#include <syncstream>#include <thread>#include <vector>usingnamespace std::chrono_literals; struct Employee{std::vector<std::string> lunch_partners;std::string id;std::mutex m;    Employee(std::string id): id(id){}std::string partners()const{std::string ret="Employee "+ id+" has lunch partners: ";for(int count{};constauto& partner: lunch_partners)            ret+=(count++?", ":"")+ partner;return ret;}}; void send_mail(Employee&, Employee&){// Simulate a time-consuming messaging operationstd::this_thread::sleep_for(1s);} void assign_lunch_partner(Employee& e1, Employee& e2){std::osyncstream synced_out(std::cout);    synced_out<< e1.id<<" and "<< e2.id<<" are waiting for locks"<<std::endl; {// Use std::scoped_lock to acquire two locks without worrying about// other calls to assign_lunch_partner deadlocking us// and it also provides a convenient RAII-style mechanism         std::scoped_lock lock(e1.m, e2.m); // Equivalent code 1 (using std::lock and std::lock_guard)// std::lock(e1.m, e2.m);// std::lock_guard<std::mutex> lk1(e1.m, std::adopt_lock);// std::lock_guard<std::mutex> lk2(e2.m, std::adopt_lock); // Equivalent code 2 (if unique_locks are needed, e.g. for condition variables)// std::unique_lock<std::mutex> lk1(e1.m, std::defer_lock);// std::unique_lock<std::mutex> lk2(e2.m, std::defer_lock);// std::lock(lk1, lk2);        synced_out<< e1.id<<" and "<< e2.id<<" got locks"<<std::endl;        e1.lunch_partners.push_back(e2.id);        e2.lunch_partners.push_back(e1.id);}     send_mail(e1, e2);    send_mail(e2, e1);} int main(){    Employee alice("Alice"), bob("Bob"), christina("Christina"), dave("Dave"); // Assign in parallel threads because mailing users about lunch assignments// takes a long timestd::vector<std::thread> threads;    threads.emplace_back(assign_lunch_partner,std::ref(alice),std::ref(bob));    threads.emplace_back(assign_lunch_partner,std::ref(christina),std::ref(bob));    threads.emplace_back(assign_lunch_partner,std::ref(christina),std::ref(alice));    threads.emplace_back(assign_lunch_partner,std::ref(dave),std::ref(bob)); for(auto& thread: threads)        thread.join();std::osyncstream(std::cout)<< alice.partners()<<'\n'<< bob.partners()<<'\n'<< christina.partners()<<'\n'<< dave.partners()<<'\n';}

Alice and Bob are waiting for locksAlice and Bob got locksChristina and Bob are waiting for locksChristina and Alice are waiting for locksDave and Bob are waiting for locksDave and Bob got locksChristina and Alice got locksChristina and Bob got locksEmployee Alice has lunch partners: Bob, ChristinaEmployee Bob has lunch partners: Alice, Dave, ChristinaEmployee Christina has lunch partners: Alice, BobEmployee Dave has lunch partners: Bob