thread_pool.hpp

//
// thread_pool.hpp
//
// exercise solution - chapter 7
// modern cpp tutorial
//
// created by changkun at changkun.de
// https://github.com/changkun/modern-cpp-tutorial/
//

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

#include <type_traits>
#include <vector>               // std::vector
#include <queue>                // std::queue
#include <memory>               // std::make_shared

#include <thread>               // std::thread
#include <mutex>                // std::mutex, std::unique_lock
#include <condition_variable>   // std::condition_variable
#include <future>               // std::future, std::packaged_task

#include <functional>           // std::function, std::bind
#include <stdexcept>            // std::runtime_error
#include <utility>              // std::move, std::forward

class ThreadPool {
public:
    
    // initialize the number of concurrency threads
    ThreadPool(size_t);
    
    // enqueue new thread task
    template<class F, class... Args>
    decltype(auto) enqueue(F&& f, Args&&... args);

    // destroy thread pool and all created threads
    ~ThreadPool();
private:
    
    // thread list, stores all threads
    std::vector< std::thread > workers;
    // queue task, the type of queue elements are functions with void return type
    std::queue< std::function<void()> > tasks;
    
    // for synchonization
    std::mutex queue_mutex;
    // std::condition_variable is a new feature from c++11,
    // it's a synchronization primitives. it can be used 
    // to block a thread or threads at the same time until
    // all of them modified condition_variable.
    std::condition_variable condition;
    bool stop;
};
 
// constructor initialize a fixed size of worker
inline ThreadPool::ThreadPool(size_t threads): stop(false) {
    // initialize worker
    for(size_t i = 0;i<threads;++i)
        // std::vector::emplace_back :
        //    append to the end of vector container
        //    this element will be constructed at the end of container, without copy and move behavior
        workers.emplace_back([this] { // the lambda express capture this, i.e. the instance of thread pool
                // avoid fake awake
                for(;;) {
                    // define function task container, return type is void
                    std::function<void()> task;

                    // critical section
                    {
                        // get mutex
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        
                        // block current thread
                        this->condition.wait(lock,
                            [this]{ return this->stop || !this->tasks.empty(); });
                        
                        // return if queue empty and task finished
                        if(this->stop && this->tasks.empty())
                            return;
                        
                        // otherwise execute the first element of queue
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                    }
                    
                    // execution
                    task();
                }
            }
        );
}

// Enqueue a new thread
// use variadic templates and tail return type 
template<class F, class... Args>
decltype(auto) ThreadPool::enqueue(F&& f, Args&&... args) {
    // deduce return type
    using return_type = typename std::invoke_result<F,Args...>::type;

    // fetch task
    auto task = std::make_shared<std::packaged_task<return_type()>>(
        std::bind(std::forward<F>(f), std::forward<Args>(args)...)
    );

    std::future<return_type> res = task->get_future();

    // critical section
    {
        std::unique_lock<std::mutex> lock(queue_mutex);

        // avoid add new thread if theadpool is destroyed
        if(stop)
            throw std::runtime_error("enqueue on stopped ThreadPool");

        // add thread to queue
        tasks.emplace([task]{ (*task)(); });
    }

    // notify a wait thread
    condition.notify_one();
    return res;
}

// destroy everything
inline ThreadPool::~ThreadPool()
{
    // critical section
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }

    // wake up all threads
    condition.notify_all();

    // let all processes into synchronous execution, use c++11 new for-loop: for(value:values)
    for(std::thread &worker: workers)
        worker.join();
}

#endif