### Timer.2 - Using a timer asynchronously

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

    #include <iostream>
    #include <asio.hpp>

Using asio's asynchronous functionality means supplying a completion token, which determines how the result will be delivered to a completion handler 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 std::error_code& /*e*/)
    {
      std::cout << "Hello, world!" << std::endl;
    }

    int main()
    {
      asio::io_context io;

      asio::steady_timer t(io, asio::chrono::seconds(5));

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

      t.async_wait(&print);

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

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

The asio::io_context::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 completion handler has returned.

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

      io.run();

      return 0;
    }

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