How to create a thread pool using boost in C++?

How do I create a thread pool using boost in C++, and how do I assign tasks to the threadpool?

Solution 1:

The process is pretty simple. First create an asio::io_service and a thread_group. Fill the thread_group with threads linked to the io_service. Assign tasks to the threads using the boost::bind function.

To stop the threads (usually when you are exiting your program) just stop the io_service and join all threads.

You should only need these headers:

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

here is an example:

/*
 * Create an asio::io_service and a thread_group (through pool in essence)
 */
boost::asio::io_service ioService;
boost::thread_group threadpool;


/*
 * This will start the ioService processing loop. All tasks 
 * assigned with ioService.post() will start executing. 
 */
boost::asio::io_service::work work(ioService);

/*
 * This will add 2 threads to the thread pool. (You could just put it in a for loop)
 */
threadpool.create_thread(
    boost::bind(&boost::asio::io_service::run, &ioService)
);
threadpool.create_thread(
    boost::bind(&boost::asio::io_service::run, &ioService)
);

/*
 * This will assign tasks to the thread pool. 
 * More about boost::bind: "http://www.boost.org/doc/libs/1_54_0/libs/bind/bind.html#with_functions"
 */
ioService.post(boost::bind(myTask, "Hello World!"));
ioService.post(boost::bind(clearCache, "./cache"));
ioService.post(boost::bind(getSocialUpdates, "twitter,gmail,facebook,tumblr,reddit"));

/*
 * This will stop the ioService processing loop. Any tasks
 * you add behind this point will not execute.
*/
ioService.stop();

/*
 * Will wait till all the threads in the thread pool are finished with 
 * their assigned tasks and 'join' them. Just assume the threads inside
 * the threadpool will be destroyed by this method.
 */
threadpool.join_all();

Source: Recipes < Asio

Solution 2:

Starting from boost 1.66.0, there is a thread_pool class:

#include <boost/asio/thread_pool.hpp>
#include <boost/asio/post.hpp>

boost::asio::thread_pool pool(4); // 4 threads
boost::asio::post(pool, [] {});
pool.join();

See the description.