ThreadPool.cpp

//
//  ThreadPool.cpp
//  Multithreading
//
//  Created by Marc Rousavy on 16.03.21.
//  Copyright © 2021 Facebook. All rights reserved.
//

#include "ThreadPool.h"
#include <sstream>
#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>
#include <unordered_map>

namespace mrousavy {
namespace multithreading {

// the constructor just launches some amount of workers
ThreadPool::ThreadPool(size_t threads): stop(false) {
  for (size_t i = 0; i < threads; ++i) {
    workers.emplace_back([this] {
      while (true) {
        std::function<void()> task;
        
        {
          std::unique_lock<std::mutex> lock(this->queue_mutex);
          this->condition.wait(lock,
                               [this]{ return this->stop || !this->tasks.empty(); });
          if(this->stop && this->tasks.empty())
            return;
          task = std::move(this->tasks.front());
          this->tasks.pop();
        }
        
        task();
      }
    });
  }
}

// add new work item to the pool
std::future<void> ThreadPool::enqueue(std::function<void()> func) {
  auto task = std::make_shared<std::packaged_task<void()>>(std::bind(std::forward<std::function<void()>>(func)));
  std::future<void> res = task->get_future();
  
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    
    // don't allow enqueueing after stopping the pool
    if (stop)
      throw std::runtime_error("enqueue on stopped ThreadPool");
    
    tasks.emplace([task](){ (*task)(); });
  }
  
  condition.notify_one();
  return res;
}

// the destructor joins all threads
ThreadPool::~ThreadPool() {
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    stop = true;
  }
  condition.notify_all();
  for (auto& worker : workers) {
    worker.join();
  }
}

}
}