#include <chrono>#include <functional>#include <iostream>#include <mutex>#include <string>#include <thread>#include <vector> struct Employee{    Employee(std::string id): id(id){}std::string id;std::vector<std::string> lunch_partners;std::mutex m;std::string output()const{std::string ret="Employee "+ id+" has lunch partners: ";for(auto n{lunch_partners.size()};constauto& partner: lunch_partners)            ret+= partner+(--n?", ":"");return ret;}}; void send_mail(Employee&, Employee&){// Simulate a time-consuming messaging operationstd::this_thread::sleep_for(std::chrono::milliseconds(696));} void assign_lunch_partner(Employee& e1, Employee& e2){staticstd::mutex io_mutex;{std::lock_guard<std::mutex> lk(io_mutex);std::cout<< e1.id<<" and "<< e2.id<<" are waiting for locks"<<std::endl;} // Use std::lock to acquire two locks without worrying about// other calls to assign_lunch_partner deadlocking us{        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 (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);// Superior solution available in C++17//  std::scoped_lock lk(e1.m, e2.m);{std::lock_guard<std::mutex> lk(io_mutex);std::cout<< 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::cout<< alice.output()<<'\n'<< bob.output()<<'\n'<< christina.output()<<'\n'<< dave.output()<<'\n';}

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