Thread pool using boost asio

In short, you need to wrap the user’s provided task with another function that will:

• Invoke the user function or callable object.
• Lock the mutex and decrement the counter.

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I may not be understanding all the requirements for this thread pool. Thus, for clarity, here is an explicit list as to what I believe are the requirements:

• The pool manages the lifetime of the threads. The user should not be able to delete threads that reside within the pool.
• The user can assign a task to the pool in a non-intrusive way.
• When a task is being assigned, if all threads in the pool are currently running other tasks, then the task is discarded.

Before I provide an implementation, there are a few key points I would like to stress:

• Once a thread has been launched, it will run until completion, cancellation, or termination. The function the thread is executing cannot be reassigned. To allow for a single thread to execute multiple functions over the course of its life, the thread will want to launch with a function that will read from a queue, such as io_service::run(), and callable types are posted into the event queue, such as from io_service::post().
• io_service::run() returns if there is no work pending in the io_service, the io_service is stopped, or an exception is thrown from a handler that the thread was running. To prevent io_serivce::run() from returning when there is no unfinished work, the io_service::work class can be used.
• Defining the task’s type requirements (i.e. the task’s type must be callable by object() syntax) instead of requiring a type (i.e. task must inherit from process), provides more flexibility to the user. It allows the user to supply a task as a function pointer or a type providing a nullary operator().

Implementation using boost::asio:

#include <boost/asio.hpp>
#include <boost/thread.hpp>

class thread_pool
{
private:
  boost::asio::io_service io_service_;
  boost::asio::io_service::work work_;
  boost::thread_group threads_;
  std::size_t available_;
  boost::mutex mutex_;
public:

  /// @brief Constructor.
  thread_pool( std::size_t pool_size )
    : work_( io_service_ ),
      available_( pool_size )
  {
    for ( std::size_t i = 0; i < pool_size; ++i )
    {
      threads_.create_thread( boost::bind( &boost::asio::io_service::run,
                                           &io_service_ ) );
    }
  }

  /// @brief Destructor.
  ~thread_pool()
  {
    // Force all threads to return from io_service::run().
    io_service_.stop();

    // Suppress all exceptions.
    try
    {
      threads_.join_all();
    }
    catch ( const std::exception& ) {}
  }

  /// @brief Adds a task to the thread pool if a thread is currently available.
  template < typename Task >
  void run_task( Task task )
  {
    boost::unique_lock< boost::mutex > lock( mutex_ );

    // If no threads are available, then return.
    if ( 0 == available_ ) return;

    // Decrement count, indicating thread is no longer available.
    --available_;

    // Post a wrapped task into the queue.
    io_service_.post( boost::bind( &thread_pool::wrap_task, this,
                                   boost::function< void() >( task ) ) );
  }

private:
  /// @brief Wrap a task so that the available count can be increased once
  ///        the user provided task has completed.
  void wrap_task( boost::function< void() > task )
  {
    // Run the user supplied task.
    try
    {
      task();
    }
    // Suppress all exceptions.
    catch ( const std::exception& ) {}

    // Task has finished, so increment count of available threads.
    boost::unique_lock< boost::mutex > lock( mutex_ );
    ++available_;
  }
};

A few comments about the implementation:

• Exception handling needs to occur around the user’s task. If the user’s function or callable object throws an exception that is not of type boost::thread_interrupted, then std::terminate() is called. This is the the result of Boost.Thread’s exceptions in thread functions behavior. It is also worth reading Boost.Asio’s effect of exceptions thrown from handlers.
• If the user provides the task via boost::bind, then the nested boost::bind will fail to compile. One of the following options is required:
  • Not support task created by boost::bind.
  • Meta-programming to perform compile-time branching based on whether or not the user’s type if the result of boost::bind so that boost::protect could be used, as boost::protect only functions properly on certain function objects.
  • Use another type to pass the task object indirectly. I opted to use boost::function for readability at the cost of losing the exact type. boost::tuple, while slightly less readable, could also be used to preserve the exact type, as seen in the Boost.Asio’s serialization example.

Application code can now use the thread_pool type non-intrusively:

void work() {};

struct worker
{
  void operator()() {};
};

void more_work( int ) {};

int main()
{ 
  thread_pool pool( 2 );
  pool.run_task( work );                        // Function pointer.
  pool.run_task( worker() );                    // Callable object.
  pool.run_task( boost::bind( more_work, 5 ) ); // Callable object.
}

The thread_pool could be created without Boost.Asio, and may be slightly easier for maintainers, as they no longer need to know about Boost.Asio behaviors, such as when does io_service::run() return, and what is the io_service::work object:

#include <queue>
#include <boost/bind.hpp>
#include <boost/thread.hpp>

class thread_pool
{
private:
  std::queue< boost::function< void() > > tasks_;
  boost::thread_group threads_;
  std::size_t available_;
  boost::mutex mutex_;
  boost::condition_variable condition_;
  bool running_;
public:

  /// @brief Constructor.
  thread_pool( std::size_t pool_size )
    : available_( pool_size ),
      running_( true )
  {
    for ( std::size_t i = 0; i < pool_size; ++i )
    {
      threads_.create_thread( boost::bind( &thread_pool::pool_main, this ) ) ;
    }
  }

  /// @brief Destructor.
  ~thread_pool()
  {
    // Set running flag to false then notify all threads.
    {
      boost::unique_lock< boost::mutex > lock( mutex_ );
      running_ = false;
      condition_.notify_all();
    }

    try
    {
      threads_.join_all();
    }
    // Suppress all exceptions.
    catch ( const std::exception& ) {}
  }

  /// @brief Add task to the thread pool if a thread is currently available.
  template < typename Task >
  void run_task( Task task )
  {
    boost::unique_lock< boost::mutex > lock( mutex_ );

    // If no threads are available, then return.
    if ( 0 == available_ ) return;

    // Decrement count, indicating thread is no longer available.
    --available_;

    // Set task and signal condition variable so that a worker thread will
    // wake up andl use the task.
    tasks_.push( boost::function< void() >( task ) );
    condition_.notify_one();
  }

private:
  /// @brief Entry point for pool threads.
  void pool_main()
  {
    while( running_ )
    {
      // Wait on condition variable while the task is empty and the pool is
      // still running.
      boost::unique_lock< boost::mutex > lock( mutex_ );
      while ( tasks_.empty() && running_ )
      {
        condition_.wait( lock );
      }
      // If pool is no longer running, break out.
      if ( !running_ ) break;

      // Copy task locally and remove from the queue.  This is done within
      // its own scope so that the task object is destructed immediately
      // after running the task.  This is useful in the event that the
      // function contains shared_ptr arguments bound via bind.
      {
        boost::function< void() > task = tasks_.front();
        tasks_.pop();

        lock.unlock();

        // Run the task.
        try
        {
          task();
        }
        // Suppress all exceptions.
        catch ( const std::exception& ) {}
      }

      // Task has finished, so increment count of available threads.
      lock.lock();
      ++available_;
    } // while running_
  }
};