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Timer.3 - Binding arguments to a completion handler

In this tutorial we will modify the program from tutorial Timer.2 so that the timer fires once a second. This will show how to pass additional parameters to your handler function.

#include <functional>
#include <iostream>
#include <boost/asio.hpp>

To implement a repeating timer using asio you need to change the timer's expiry time in your completion handler, and to then start a new asynchronous wait. Obviously this means that the completion handler will need to be able to access the timer object. To this end we add two new parameters to the print function:

at the end of the parameter list.

void print(const boost::system::error_code& /*e*/,
    boost::asio::steady_timer* t, int* count)

As mentioned above, this tutorial program uses a counter to stop running when the timer fires for the sixth time. However you will observe that there is no explicit call to ask the io_context to stop. Recall that in tutorial Timer.2 we learnt that the boost::asio::io_context::run() function completes when there is no more "work" to do. By not starting a new asynchronous wait on the timer when count reaches 5, the io_context will run out of work and stop running.

  if (*count < 5)
    std::cout << *count << std::endl;

Next we move the expiry time for the timer along by one second from the previous expiry time. By calculating the new expiry time relative to the old, we can ensure that the timer does not drift away from the whole-second mark due to any delays in processing the handler.

    t->expires_at(t->expiry() + boost::asio::chrono::seconds(1));

Then we start a new asynchronous wait on the timer. As you can see, the std::bind function is used to associate the extra parameters with your completion handler. The steady_timer::async_wait() function expects a handler function (or function object) with the signature void(const boost::system::error_code&). Binding the additional parameters converts your print function into a function object that matches the signature correctly.

In this example, the boost::asio::placeholders::error argument to std::bind is a named placeholder for the error object passed to the handler. When initiating the asynchronous operation, and if using std::bind, you must specify only the arguments that match the handler's parameter list. In tutorial Timer.4 you will see that this placeholder may be elided if the parameter is not needed by the completion handler.

          boost::asio::placeholders::error, t, count));

int main()
  boost::asio::io_context io;

A new count variable is added so that we can stop the program when the timer fires for the sixth time.

  int count = 0;
  boost::asio::steady_timer t(io, boost::asio::chrono::seconds(1));

As in Step 4, when making the call to steady_timer::async_wait() from main we bind the additional parameters needed for the print function.

        boost::asio::placeholders::error, &t, &count));;

Finally, just to prove that the count variable was being used in the print handler function, we will print out its new value.

  std::cout << "Final count is " << count << std::endl;

  return 0;

See the full source listing

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