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### Timer.1 - Using a timer synchronously

This tutorial program introduces asio by showing how to perform a blocking wait on a timer.

We start by including the necessary header files.

All of the asio classes can be used by simply including the `"asio.hpp"` header file.

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

Since this example uses timers, we need to include the appropriate Boost.Date_Time header file for manipulating times.

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

All programs that use asio need to have at least one io_service object. This class provides access to I/O functionality. We declare an object of this type first thing in the main function.

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

Next we declare an object of type boost::asio::deadline_timer. The core asio classes that provide I/O functionality (or as in this case timer functionality) always take a reference to an io_service as their first constructor argument. The second argument to the constructor sets the timer to expire 5 seconds from now.

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

In this simple example we perform a blocking wait on the timer. That is, the call to deadline_timer::wait() will not return until the timer has expired, 5 seconds after it was created (i.e. not from when the wait starts).

A deadline timer is always in one of two states: "expired" or "not expired". If the deadline_timer::wait() function is called on an expired timer, it will return immediately.

      t.wait();

Finally we print the obligatory `"Hello, world!"` message to show when the timer has expired.

      std::cout << "Hello, world!" << std::endl;

      return 0;
    }

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Next: Timer.2 - Using a timer asynchronously