condition variable wait unlocks mutex c++ code example

Example: condition variable wait unlocks mutex c++

std::mutex mutex;
std::queue buffer;              
std::condition_variable buffer_cond;

void data_preparation_thread()
{
    while(has_data_to_prepare())                //--  (1)    
    {
      buffer_data data = prepare_data();
      std::lock_quard lock(mutex);  //--  (2) 
      buffer.push(data);                        
      buffer_cond.notify_one();                 //-- (3)  
    }
}

void data_processing_thread()
{
    while(true)
    {
      std::unique_lock lock(mutex);              //-- (4)  
      buffer_cond.wait(lock, []{return ! buffer.empty()})    //-- (5)   
      buffer_data data = buffer.front();
      buffer.pop();
      lock.unlock();                                         //-- (6)   
      process(data);
      if(is_last_data_entry(data)) 
          break;         
    }  
}