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In this tutorial we will see how to use a class member function as a callback handler. The program should execute identically to the tutorial program from tutorial Timer.3.
#include <iostream> #include <asio.hpp> #include <boost/bind.hpp> #include <boost/date_time/posix_time/posix_time.hpp>
        Instead of defining a free function print
        as the callback 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_service 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(asio::io_service& io) : timer_(io, boost::posix_time::seconds(1)), count_(0) {
        The boost::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, boost::bind
        converts our callback handler (now a member function) into a function object
        that can be invoked as though it has the signature void(const asio::error_code&).
      
        You will note that the 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(boost::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_.expires_at() + boost::posix_time::seconds(1)); timer_.async_wait(boost::bind(&printer::print, this)); } } private: asio::deadline_timer timer_; int count_; };
        The main function is much
        simpler than before, as it now declares a local printer
        object before running the io_service as normal.
      
int main() { asio::io_service io; printer p(io); io.run(); return 0; }
See the full source listing
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