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### Timer.4 - Using a member function as a completion handler

In this tutorial we will see how to use a class member function as a completion handler. The program should execute identically to the tutorial program from tutorial Timer.3.

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

Instead of defining a free function `print` as the completion handler, as we did in the earlier tutorial programs, we now define a class called `printer`.

    class printer
    {
    public:

The constructor of this class will take a reference to the io_context object and use it when initialising the `timer_` member. The counter used to shut down the program is now also a member of the class.

      printer(boost::asio::io_context& io)
        : timer_(io, boost::asio::chrono::seconds(1)),
          count_(0)
      {

The `std::bind` function works just as well with class member functions as with free functions. Since all non-static class member functions have an implicit `this` parameter, we need to bind `this` to the function. As in tutorial Timer.3, `std::bind` converts our completion handler (now a member function) into a function object that can be invoked as though it has the signature `void(const boost::system::error_code&)`.

You will note that the boost::asio::placeholders::error placeholder is not specified here, as the `print` member function does not accept an error object as a parameter.

        timer_.async_wait(std::bind(&printer::print, this));
      }

In the class destructor we will print out the final value of the counter.

      ~printer()
      {
        std::cout << "Final count is " << count_ << std::endl;
      }

The `print` member function is very similar to the `print` function from tutorial Timer.3, except that it now operates on the class data members instead of having the timer and counter passed in as parameters.

      void print()
      {
        if (count_ < 5)
        {
          std::cout << count_ << std::endl;
          ++count_;

          timer_.expires_at(timer_.expiry() + boost::asio::chrono::seconds(1));
          timer_.async_wait(std::bind(&printer::print, this));
        }
      }

    private:
      boost::asio::steady_timer timer_;
      int count_;
    };

The `main` function is much simpler than before, as it now declares a local `printer` object before running the io_context as normal.

    int main()
    {
      boost::asio::io_context io;
      printer p(io);
      io.run();

      return 0;
    }

See the full source listing

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Next: Timer.5 - Synchronising completion handlers in multithreaded programs