How to limit the number of running instances in C++

Question

I have a c++ class that allocates a lot of memory. It does this by calling a third-party library that is designed to crash if it cannot allocate the memory, and sometimes my

Answer 1

Here's a simplistic way to implement your own 'semaphore' (since I don't think the standard library or boost have one). This chooses a 'cooperative' approach and workers will wait for each other:

#include 
#include 

using namespace boost;
using namespace boost::phoenix::arg_names;

void the_work(int id)
{
    static int running = 0;
    std::cout << "worker " << id << " entered (" << running << " running)\n";

    static mutex mx;
    static condition_variable cv;

    // synchronize here, waiting until we can begin work
    {
        unique_lock lk(mx);
        cv.wait(lk, phoenix::cref(running) < 3);
        running += 1;
    }

    std::cout << "worker " << id << " start work\n";
    this_thread::sleep_for(chrono::seconds(2));
    std::cout << "worker " << id << " done\n";

    // signal one other worker, if waiting
    {
        lock_guard lk(mx);
        running -= 1;
        cv.notify_one(); 
    }
}

int main()
{
    thread_group pool;

    for (int i = 0; i < 10; ++i)
        pool.create_thread(bind(the_work, i));

    pool.join_all();
}

Now, I'd say it's probably better to have a dedicated pool of n workers taking their work from a queue in turns:

#include 
#include 
#include 

using namespace boost;
using namespace boost::phoenix::arg_names;

class thread_pool
{
  private:
      mutex mx;
      condition_variable cv;

      typedef function job_t;
      std::deque _queue;

      thread_group pool;

      boost::atomic_bool shutdown;
      static void worker_thread(thread_pool& q)
      {
          while (auto job = q.dequeue())
              (*job)();
      }

  public:
      thread_pool() : shutdown(false) {
          for (unsigned i = 0; i < boost::thread::hardware_concurrency(); ++i)
              pool.create_thread(bind(worker_thread, ref(*this)));
      }

      void enqueue(job_t job) 
      {
          lock_guard lk(mx);
          _queue.push_back(std::move(job));

          cv.notify_one();
      }

      optional dequeue() 
      {
          unique_lock lk(mx);
          namespace phx = boost::phoenix;

          cv.wait(lk, phx::ref(shutdown) || !phx::empty(phx::ref(_queue)));

          if (_queue.empty())
              return none;

          auto job = std::move(_queue.front());
          _queue.pop_front();

          return std::move(job);
      }

      ~thread_pool()
      {
          shutdown = true;
          {
              lock_guard lk(mx);
              cv.notify_all();
          }

          pool.join_all();
      }
};

void the_work(int id)
{
    std::cout << "worker " << id << " entered\n";

    // no more synchronization; the pool size determines max concurrency
    std::cout << "worker " << id << " start work\n";
    this_thread::sleep_for(chrono::seconds(2));
    std::cout << "worker " << id << " done\n";
}

int main()
{
    thread_pool pool; // uses 1 thread per core

    for (int i = 0; i < 10; ++i)
        pool.enqueue(bind(the_work, i));
}

PS. You can use C++11 lambdas instead boost::phoenix there if you prefer.