cv.cpp

// condition_variable::wait (with predicate)
#include <iostream>           // std::cout
#include <thread>             // std::thread, std::this_thread::yield
#include <mutex>              // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable

std::mutex mtx;
std::condition_variable cv;

int cargo = 0;
bool shipment_available() {return cargo!=0;}

void consume (int n) {
  for (int i=0; i<n; ++i) {
    std::unique_lock<std::mutex> lck(mtx);
    cv.wait(lck,shipment_available);
    // consume:
    std::cout << cargo << '\n';
    cargo=0;
  }
}

void produce (int n) {
  for (int i=0; i<10; ++i) {
    while (shipment_available()) std::this_thread::yield();
    std::unique_lock<std::mutex> lck(mtx);
    cargo = i+1;
    std::cout << i << ": sent " << cargo << std::endl;
    cv.notify_one();
  }
}

int main ()
{
  std::thread producer_thread (produce, 10);
  std::thread consumer_thread (consume,10);
  // produce 10 items when needed:
  // for (int i=0; i<10; ++i) {
  //   while (shipment_available()) std::this_thread::yield();
  //   std::unique_lock<std::mutex> lck(mtx);
  //   cargo = i+1;
  //   std::cout << i << ": sent " << cargo << std::endl;
  //   cv.notify_one();
  // }

  producer_thread.join();
  consumer_thread.join();

  return 0;
}