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### Timer.2 - Using a timer asynchronously

This tutorial program demonstrates how to use asio's asynchronous callback functionality by modifying the program from tutorial Timer.1 to perform an asynchronous wait on the timer.

    #include <iostream>
    #include <boost/asio.hpp>
    #include <boost/date_time/posix_time/posix_time.hpp>

Using asio's asynchronous functionality means having a callback function that will be called when an asynchronous operation completes. In this program we define a function called `print` to be called when the asynchronous wait finishes.

    void print(const boost::system::error_code& /*e*/)
    {
      std::cout << "Hello, world!\n";
    }

    int main()
    {
      boost::asio::io_service io;

      boost::asio::deadline_timer t(io, boost::posix_time::seconds(5));

Next, instead of doing a blocking wait as in tutorial Timer.1, we call the boost::asio::deadline_timer::async_wait() function to perform an asynchronous wait. When calling this function we pass the `print` callback handler that was defined above.

      t.async_wait(print);

Finally, we must call the boost::asio::io_service::run() member function on the io_service object.

The asio library provides a guarantee that callback handlers will only be called from threads that are currently calling boost::asio::io_service::run(). Therefore unless the boost::asio::io_service::run() function is called the callback for the asynchronous wait completion will never be invoked.

The boost::asio::io_service::run() function will also continue to run while there is still "work" to do. In this example, the work is the asynchronous wait on the timer, so the call will not return until the timer has expired and the callback has completed.

It is important to remember to give the io_service some work to do before calling boost::asio::io_service::run(). For example, if we had omitted the above call to boost::asio::deadline_timer::async_wait(), the io_service would not have had any work to do, and consequently boost::asio::io_service::run() would have returned immediately.

      io.run();

      return 0;
    }

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