Bad thread pool example from C++ Concurrency in Action, it's a busy-loop.

badthreadpool.cc

#include <atomic>
#include <mutex>
#include <condition_variable>
#include <memory>
#include <queue>
#include <thread>
#include <unistd.h>

// Listing 4.5
template<typename T>
class threadsafe_queue
{
private:
    mutable std::mutex mut;
    std::queue<T> data_queue;
    std::condition_variable data_cond;
public:
    threadsafe_queue()
    {}
    threadsafe_queue(threadsafe_queue const& other)
    {
        std::lock_guard<std::mutex> lk(other.mut);
        data_queue=other.data_queue;
    }

    void push(T new_value)
    {
        std::lock_guard<std::mutex> lk(mut);
        data_queue.push(new_value);
        data_cond.notify_one();
    }

    void wait_and_pop(T& value)
    {
        std::unique_lock<std::mutex> lk(mut);
        data_cond.wait(lk,[this]{return !data_queue.empty();});
        value=data_queue.front();
        data_queue.pop();
    }

    std::shared_ptr<T> wait_and_pop()
    {
        std::unique_lock<std::mutex> lk(mut);
        data_cond.wait(lk,[this]{return !data_queue.empty();});
        std::shared_ptr<T> res(std::make_shared<T>(data_queue.front()));
        data_queue.pop();
        return res;
    }

    bool try_pop(T& value)
    {
        std::lock_guard<std::mutex> lk(mut);
        if(data_queue.empty())
            return false;
        value=data_queue.front();
        data_queue.pop();
        return true;
    }

    std::shared_ptr<T> try_pop()
    {
        std::lock_guard<std::mutex> lk(mut);
        if(data_queue.empty())
            return std::shared_ptr<T>();
        std::shared_ptr<T> res(std::make_shared<T>(data_queue.front()));
        data_queue.pop();
        return res;
    }

    bool empty() const
    {
        std::lock_guard<std::mutex> lk(mut);
        return data_queue.empty();
    }
};

// Page 248, above listing 8.4
class join_threads
{
  std::vector<std::thread>& threads;
 public:
  explicit join_threads(std::vector<std::thread>& threads_):
    threads(threads_)
  {}

  ~join_threads()
  {
    for(unsigned long i=0;i<threads.size();++i)
    {
      if(threads[i].joinable())
        threads[i].join();
    }
  }
};

// Listing 9.1
class thread_pool
{
    std::atomic_bool done;
    threadsafe_queue<std::function<void()> > work_queue;
    std::vector<std::thread> threads;
    join_threads joiner;

    void worker_thread()
    {
        while(!done)
        {
            std::function<void()> task;
            if (work_queue.try_pop(task))
            {
                task();
            }
            else
            {
                std::this_thread::yield();
            }
        }
    }
public:
    thread_pool():
        done(false),joiner(threads)
    {
        unsigned const thread_count=std::thread::hardware_concurrency();
        try
        {
            for(unsigned i=0;i<thread_count;++i)
            {
                threads.push_back(
                    std::thread(&thread_pool::worker_thread,this));
            }
        }
        catch(...)
        {
            done=true;
            throw;
        }
    }

    ~thread_pool()
    {
        done=true;
    }

    template<typename FunctionType>
    void submit(FunctionType f)
    {
        work_queue.push(std::function<void()>(f));
    }
};

int main()
{
  thread_pool p;
  sleep(60);
}