...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/bind.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_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 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_ << 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(boost::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 handlers in multithreaded programs