Simple Multithread Timer

Question

This is a very basic timer that can support multithreading with std::thread and std::chrono.

The timer has the classic functions: start() and stop().

The start() method creates an independent thread (if multithread support is enabled), then sleep the thread for a given Interval, then execute Timeout function. This method sets the running flag to true.

If singleShot flag is not enabled the sleepThenTimeout process is called while a running flag is true.

If multithread support is not enabled, the current thread is sleep.

The stop() method just sets the running flag to false and join the thread.

My doubt is about threat-safety. I've just started to learn how multithreading works, so I'm not sure if I must use mutexes or something like that.

Any other type of feedback are welcome!

Timer.h

#ifndef TIMER_H
#define TIMER_H
#include <thread>
#include <chrono>
class Timer
{
public:
 typedef std::chrono::milliseconds Interval;
 typedef std::function<void(void)> Timeout;
 Timer(const Timeout &timeout);
 Timer(const Timeout &timeout,
 const Interval &interval,
 bool singleShot = true);
 void start(bool multiThread = false);
 void stop();
 bool running() const;
 void setSingleShot(bool singleShot);
 bool isSingleShot() const;
 void setInterval(const Interval &interval);
 const Interval &interval() const;
 void setTimeout(const Timeout &timeout);
 const Timeout &timeout() const;
private:
 std::thread _thread;
 bool _running = false;
 bool _isSingleShot = true;
 Interval _interval = Interval(0);
 Timeout _timeout = nullptr;
 void _temporize();
 void _sleepThenTimeout();
};
#endif // TIMER_H

Timer.cpp

#include "Timer.h"
Timer::Timer(const Timeout &timeout)
 : _timeout(timeout)
{
}
Timer::Timer(const Timer::Timeout &timeout,
 const Timer::Interval &interval,
 bool singleShot)
 : _isSingleShot(singleShot),
 _interval(interval),
 _timeout(timeout)
{
}
void Timer::start(bool multiThread)
{
 if (this->running() == true)
 return;
 _running = true;
 if (multiThread == true) {
 _thread = std::thread(
 &Timer::_temporize, this);
 }
 else{
 this->_temporize();
 }
}
void Timer::stop()
{
 _running = false;
 _thread.join();
}
bool Timer::running() const
{
 return _running;
}
void Timer::setSingleShot(bool singleShot)
{
 if (this->running() == true)
 return;
 _isSingleShot = singleShot;
}
bool Timer::isSingleShot() const
{
 return _isSingleShot;
}
void Timer::setInterval(const Timer::Interval &interval)
{
 if (this->running() == true)
 return;
 _interval = interval;
}
const Timer::Interval &Timer::interval() const
{
 return _interval;
}
void Timer::setTimeout(const Timeout &timeout)
{
 if (this->running() == true)
 return;
 _timeout = timeout;
}
const Timer::Timeout &Timer::timeout() const
{
 return _timeout;
}
void Timer::_temporize()
{
 if (_isSingleShot == true) {
 this->_sleepThenTimeout();
 }
 else {
 while (this->running() == true) {
 this->_sleepThenTimeout();
 }
 }
}
void Timer::_sleepThenTimeout()
{
 std::this_thread::sleep_for(_interval);
 if (this->running() == true)
 this->timeout()();
}

main.cpp

#include <iostream>
#include "Timer.h"
using namespace std;
int main(void)
{
 Timer tHello([]()
 {
 cout << "Hello!" << endl;
 });
 tHello.setSingleShot(false);
 tHello.setInterval(Timer::Interval(1000));
 tHello.start(true);
 Timer tStop([&]()
 {
 tHello.stop();
 });
 tStop.setSingleShot(true);
 tStop.setInterval(Timer::Interval(3000));
 tStop.start();
 return 0;
}

• Loki Astari: Why would you use up all the resources required for a thread when you can set SIGALARM?
• Mario Frescas: As far I know SIGALRM is not cross-plataform.
• Loki Astari: What platform are you thinking about that does not have sig alarm but does have a threading model?
• Mario Frescas: Sorry, my bad! I've chosen to use threads because I want to learn about threads. I've also could use Qt's Timer but that is not the way.

Answer 1

Basically you are working with these class members inside the thread functions _temporize and _sleepThenTimeout.

• timeout, _isSingleShort and _interval, these three cannot be changed after the start function is called, so it is safe to use them.
• _running on the other hand can be read/write by both threads. In reality it might not cause any problem as assignment to bool is atomic (on most architectures), but to be 100% safe you can use std::atomic<bool>.

Another important thing to keep in mind is that while timer class itself is thread safe, it is responsibility of the user of this class to make sure that the timer call back function (_timeout) is thread-safe by it self, as it will be executed in a separate thread.

• Mario Frescas: What do you think about replace if (this->running() == true) return; with an assert?
• Ammar: Yes, assert will be better suited here, as it is better to let the caller know of the problem, rather than failing silently. But assert or the if statement won't affect the thread safety.

Answer 2

I changed the testing part in main to

int cntHello = 0;
int cntStop = 0;
Timer tHello([&]()
{
 cntHello++;
});
tHello.setSingleShot(false);
tHello.setInterval(Timer::Interval(1000));
tHello.start(true);
Timer tStop([&]()
{
 ++cntStop;
 tHello.stop();
});
tStop.setSingleShot(true);
tStop.setInterval(Timer::Interval(3001));
tStop.start(true);
// TODO why could join tHello at this point?
// tHello.join();
tStop.join();
tHello.join();

First change is the during creating the Timer for tHello. I add '&' between '[]' otherwise I cout not use cntHello inside the function. Why? (The counters are used later for checking how often the functions are called.)

Second I use tStop allthough as multithread and wait after starting it until the threads have been finshed.(by using join) For that I wrote methods join() and joinable() for the class Timer with only the the methods for _thread. This works if I join for tStop first. If I join tHello first I got an exception. Can you explain why?