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/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(asio::io_context& io) : timer_(io, 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 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_.expiry() + asio::chrono::seconds(1)); timer_.async_wait(boost::bind(&printer::print, this)); } } private: 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() { asio::io_context io; printer p(io); io.run(); return 0; }
See the full source listing
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