In the example here, it is necessary for the function print to have the signature void(const boost::system::error_code&)
The boost::asio::deadline_timer::async_wait() function expects a handler function (or function object) with the signature void(const boost::system::error_code&). Binding the additional parameters converts your print function into a function object that matches the signature correctly.
Timer.3 - Binding arguments to a handler
//
// timer.cpp
// ~~~~~~~~~
//
// Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
void print(const boost::system::error_code& /*e*/,
boost::asio::deadline_timer* t, int* count)
{
if (*count < 5)
{
std::cout << *count << "
";
++(*count);
t->expires_at(t->expires_at() + boost::posix_time::seconds(1));
t->async_wait(boost::bind(print,
boost::asio::placeholders::error, t, count));
}
}
int main()
{
boost::asio::io_service io;
int count = 0;
boost::asio::deadline_timer t(io, boost::posix_time::seconds(1));
t.async_wait(boost::bind(print, boost::asio::placeholders::error, &t, &count));
io.run();
std::cout << "Final count is " << count << "
";
return 0;
}
According to the discussion in this example, the same is true, except it is somehow disappeared from the member function definition.
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.4 - Using a member function as a handler
//
// timer.cpp
// ~~~~~~~~~
//
// Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
class printer
{
public:
printer(boost::asio::io_service& io)
: timer_(io, boost::posix_time::seconds(1)),
count_(0)
{
timer_.async_wait(boost::bind(&printer::print, this));
}
~printer()
{
std::cout << "Final count is " << count_ << "
";
}
void print()
{
if (count_ < 5)
{
std::cout << count_ << "
";
++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_;
};
int main()
{
boost::asio::io_service io;
printer p(io);
io.run();
return 0;
}
This makes no sense to me. I looked at the documentation for boost::bind Boost: bind.hpp documentation but saw nothing that helped me understand this.
Can someone please explain how the use of boost::bind on a member function which only takes the implicit this pointer results in a function with the signature void(const boost::system::error_code&)?