[Multiple producer - one consumer] with the modern C++

producer-consumer.cpp

#include <thread>
#include <iostream>
#include <queue>
#include <mutex>
#include <algorithm>
#include <condition_variable>
#include <atomic>
#include <random>

std::mutex mtx;
std::condition_variable condvar;
std::atomic<int> wip; // workers in progress

std::queue<int> queue;

void producer(int n, std::mt19937 &generator)
{
    for (int i = 0; i != n; ++i)
    {
        auto sleep_time = 100 + generator() % 1000;
        {
            std::lock_guard<std::mutex> lg(mtx);
            queue.push(i);
            std::cout << "[tid=" << std::this_thread::get_id() << "] pushing " << i << " add sleep for " << sleep_time << "ms" << std::endl;
        }
        condvar.notify_all();
        // sleep
        std::this_thread::sleep_for(std::chrono::milliseconds(sleep_time));
    }
    --wip;
    condvar.notify_all();
}

void consumer()
{
    while (true)
    {
        std::unique_lock<std::mutex> ul(mtx);
        condvar.wait(ul, [] { return !wip || !queue.empty(); }); // spurious wakeup protection, i.e:
        //while (wip || queue.empty())
        //    condvar.wait(ul);
        while (!queue.empty())
        {
            std::cout << "[tid=" << std::this_thread::get_id() << "] consuming " << queue.front() << std::endl;
            queue.pop();
        }
        if (wip == 0)
            break;
    }
}

int main()
{
    const int max_tasks = 10; // how many int do we need from the each producer?

    // Random generator
    std::mt19937 generator((unsigned int)std::chrono::system_clock::now().time_since_epoch().count());

    std::vector<std::thread *> threads(std::thread::hardware_concurrency()); // threads == vCPUs
    // start all
    std::transform(threads.begin(), threads.end(), threads.begin(), [&generator](auto *thr) {
        ++wip;
        return new std::thread(producer, max_tasks, std::ref(generator));
    });
    std::thread cons(consumer);
    // join all
    std::for_each(threads.begin(), threads.end(), [](auto *thr) { thr->join(); delete thr; });
    cons.join();

    std::cout << "Completed!" << std::endl;
}