...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 <boost/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(boost::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 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(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_ << "\n"; }
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_ << "\n"; ++count_; timer_.expires_at(timer_.expires_at() + boost::posix_time::seconds(1)); timer_.async_wait(boost::bind(&printer::print, this)); } } private: boost::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() { boost::asio::io_service io; printer p(io); io.run(); return 0; }
See the full source listing
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Previous: Timer.3 - Binding arguments to a handler
Next: Timer.5 - Synchronising handlers in multithreaded programs