### 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 <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 asio::error_code& /*e*/)
    {
      std::cout << "Hello, world!\n";
    }

    int main()
    {
      asio::io_service io;

      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 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 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 asio::io_service::run(). Therefore unless the asio::io_service::run() function is called the callback for the asynchronous wait completion will never be invoked.

The 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 asio::io_service::run(). For example, if we had omitted the above call to asio::deadline_timer::async_wait(), the io_service would not have had any work to do, and consequently asio::io_service::run() would have returned immediately.

      io.run();

      return 0;
    }

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