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( const auto& partner : lunch_partners )

ret += partner + " ";

return ret;

}

{

// simulate a time-consuming messaging operation

std::this_thread::sleep_for(std::chrono::seconds(1));

{

static std::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);

{

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);

{

Employee alice("alice"), bob("bob"), christina("christina"), dave("dave");

// assign in parallel threads because mailing users about lunch assignments

// takes a long time

std::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';