#include <mutex>
#include <thread>
#include <iostream>
#include <vector>
#include <functional>
#include <chrono>
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( const auto& partner : lunch_partners )
ret += partner + " ";
return ret;
}
};
void send_mail(Employee &, Employee &)
{
// simulate a time-consuming messaging operation
std::this_thread::sleep_for(std::chrono::seconds(1));
}
void assign_lunch_partner(Employee &e1, Employee &e2)
{
{
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);
// Equivalent code (if unique_lock's is 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);
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);
