c++ saving bound object and using it after asio - c++

Question

I'm trying to save the result of bind to std:function, then pass it as parameter to another function, and store it as data member. Then I use asio async_wait, but when i return from the wait, and try to operate the function i saved i get segmentation fault. any Idea why?
#include <memory>
#include <iostream>
#include <asio/io_service.hpp>
#include <functional>
#include <asio/deadline_timer.hpp>
using namespace std;
typedef std::function<void (const std::error_code& error)> TM_callback;
class Timer {
public:
Timer(asio::io_service& io_service) :_timer(io_service) {}
void start(TM_callback cb) {
_cb = cb;
_timer.expires_from_now(boost::posix_time::milliseconds(1000));
TM_callback timeoutFunc = std::bind(&Timer::onTimeout, this, std::placeholders::_1);
_timer.async_wait(timeoutFunc);
}
private:
void onTimeout(const std::error_code& error) {
(_cb)(error); // <-- here i get segmentation fault
}
TM_callback _cb;
asio::deadline_timer _timer;
};
class COL {
public:
COL(asio::io_service& io_service): _inTimer(io_service){}
void startInTimer() {
TM_callback cb = std::bind(&COL::onInTimeout, this, std::placeholders::_1);
_inTimer.start(cb);
}
private:
void onInTimeout(const std::error_code& error) {cout<<error.message();}
Timer _inTimer;
};
int main()
{
asio::io_service io_service;
COL col(io_service);
col.startInTimer();
return 0;
}

Answer 1

Ok, the most likely problem is in the code you don't show. As you can see #m.s. didn't "imagine" your problem. He forgot the io_service::run() too:
int main() {
asio::io_service io_service;
COL col(io_service);
col.startInTimer();
io_service.run();
}
Still no problem. Live On Coliru
The problem starts when inTimer is not guaranteed to live until the completion handler is executed:
int main() {
asio::io_service io_service;
{
COL col(io_service);
col.startInTimer();
}
io_service.run();
}
Now you have Undefined Behaviour: Live On Coliru
Solution
The easiest solution is to make the COL (what is that?) object live long enough. The more structural/idiomatic way would to let the bind keep the Timer object alive, e.g. using a shared_ptr:
Live On Coliru
#include <iostream>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/make_shared.hpp>
#include <boost/asio.hpp>
using namespace std;
typedef std::function<void(const boost::system::error_code &error)> TM_callback;
namespace asio = boost::asio;
class Timer : public boost::enable_shared_from_this<Timer> {
public:
Timer(asio::io_service &io_service) : _timer(io_service) {}
void start(TM_callback cb) {
_cb = cb;
_timer.expires_from_now(boost::posix_time::milliseconds(1000));
TM_callback timeoutFunc = boost::bind(&Timer::onTimeout, shared_from_this(), boost::asio::placeholders::error);
_timer.async_wait(timeoutFunc);
}
private:
void onTimeout(const boost::system::error_code &error) {
(_cb)(error);
}
TM_callback _cb;
asio::deadline_timer _timer;
};
class COL : public boost::enable_shared_from_this<COL> {
public:
COL(asio::io_service &io_service) : _svc(io_service) {}
void startInTimer() {
TM_callback cb = boost::bind(&COL::onInTimeout, shared_from_this(), boost::asio::placeholders::error);
boost::shared_ptr<Timer> _inTimer = boost::make_shared<Timer>(_svc);
_inTimer->start(cb);
}
private:
void onInTimeout(const boost::system::error_code &error) { cout << error.message(); }
asio::io_service& _svc;
};
int main() {
asio::io_service io_service;
{
boost::make_shared<COL>(io_service)->startInTimer();
}
io_service.run();
}
Note that this subtly also fixes the problem that more than one timer couldn't be in flight at a give time (scheduling a new timer would cancel the pending one).