To have a thread wait for an event to happen like a condition to become true or a task to be completed by another thread.

Say 2 threads:

• thread1 performs some actions, then sets a flag.
• thread2 is waiting for this flag to be set.

#include<iostream>
#include<thread>
#include<mutex>
 
class Application
{
 std::mutex m_mutex;
 bool m_bDataLoaded;
 
public:
  Application()
  {
    m_bDataLoaded = false;
  }
  void loadData()
  {
    // Make This Thread sleep for 1 Second.
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    std::cout<<"Loading Data from XML"<<std::endl;
 
    // Lock The Data structure.
    std::lock_guard<std::mutex> guard(m_mutex);
 
    // Set the flag to true, means data is loaded.
    m_bDataLoaded = true;
  }
 
  void mainTask()
  {
    std::cout<<"Do Some Handshaking"<<std::endl;
 
    // Acquire the Lock.
    m_mutex.lock();
 
    // Check if flag is set to true or not.
    while(m_bDataLoaded != true)
    {
      // Release the lock.
      m_mutex.unlock();
 
      // Sleep for 100 milliseconds.
      std::this_thread::sleep_for(std::chrono::milliseconds(100));
 
      // Acquire the lock
      m_mutex.lock();
    }
 
    // Release the lock.
    m_mutex.unlock();
 
    // Doc processing on loaded Data.
    std::cout<<"Do Processing On loaded Data"<<std::endl;
 }
};
 
int main()
{
  Application app;
  std::thread thread_1(&Application::mainTask, &app);
  std::thread thread_2(&Application::loadData, &app);
  thread_2.join();
  thread_1.join();
  return 0;
}