How do you start more than one thread in C++

Question

I have the following method on a class

void Listener::Start()
{
    Logger logger;
    std::string logMessage("Starting '" + to_utf8string(GetName()) + "' Listener");
    http_listener httpListener(GetUri());
    std::string listenerName(to_utf8string(name));

    logger.log(logMessage);

    // listener recieves a GET request.
    httpListener.support(methods::GET, [listenerName](http_request request)
    {
        Logger logger;
        std::string logMessage("GET request recieved from " + listenerName);

        logger.log(logMessage);

        // dummy line just till routing is dealt with
        request.reply(status_codes::OK, logMessage);

    });

    // open listener and route request 
    httpListener
        .open()
        .then([&httpListener,listenerName](){ 
            Logger logger;
            std::string logMessage(listenerName + "started");

            logger.log(logMessage);

        }).wait();

    // JUST WAIT - we do not want the application to stop
    while (true);

}

Now I do not know how many threads there are - it is basically just the number of records read from a database table.

for each (Listener l in ls.Select(m.GetId()))
{
    l.Start();
}

Only the first thread is started and runs, Which is kind of logical in that the only thing stopping the thread from running is a permanent loop.

If however it is run this way;

    std::vector<thread> listener_threads;

    for each (Listener l in ls.Select(m.GetId()))
    {
        listener_threads.push_back(thread{ &Listener::Start, &l });
    }

None of the threads seem to be running - none of the listeners reply to any request.

So the question is how can you run an indeterminate number of threads in a C++ application.

@JoachimPileborg, I think that might be the MSVC extension one. Considering this is C++11, though, there's no need for it. — chris, Apr 05 '15 at 15:37

Paul S Chapman · Accepted Answer · 2015-04-05T19:44:57.630

The way to do this is first collect all the listener instances in a vector. Then loop through this vector creating another vector with the actual threads.

// collect all the listeners into a vector

for (auto& m : rm.Select()) {
    for (auto& l : ls.Select(m.GetId())) {
        lvector.push_back(l);
    }
}

// now create threads for each listener
for (auto& lstnr : lvector)
{
    listener_threads.push_back(std::thread{ &Listener::Start, &lstnr });
}

// now join with this thread.
for (auto& t : listener_threads)
{
    t.join();
}

As I have a while(true) at the bottom of each thread - then the application just continues to run while listening on all the required domains.

I think there maybe an issue with scope as the lvector needs to be outside the for loops and they are all added.

How do you start more than one thread in C++

1 Answers1