...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 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
Return to the tutorial index
Previous: Timer.3 - Binding arguments to a completion handler
Next: Timer.5 - Synchronising completion handlers in multithreaded programs