use a callback as a parameter - c++

Question

I have the following class:
class Timer
{
public:
Timer(){};
~Timer(){};
void timer(int);
//...
private:
//...
};
My function timer(int value) is a callback that I use in glutTimerFunc(), inside the function timer(int value) i need to use the function timer again, something like this:
void Timer::timer(int value)
{
//...
glutTimerFunc(state->getTimer(), this->timer, 0);
}
How can i do it without using a static function?

Answer 1

you'll need a global dispatcher that turns the int passed to glutTimerFunc into a c++ callback (member function, lambda, etc)
something like this
struct timer_dispatch
{
using callback_t = std::function<void()>;
int start_timer(int msecs, callback_t callback) {
std::unique_lock<std::mutex> lock(_mutex);
int ident = _next_id++;
_callbacks.emplace(ident, std::move(callback));
glutTimerFunc(msecs, &timer_dispatch::dispatch_timer, ident);
return ident;
}
// implement similar function for stop timer - don't forget the mutex
void stop_timer(int ident) {
std::unique_lock<std::mutex> lock(_mutex);
_callbacks.erase(ident);
}
static timer_dispatch& instance() {
static timer_dispatch _;
return _;
}
private:
// private constructor ensures use via the instance() static method;
timer_dispatch() = default;
static void dispatch_timer(int ident) {
auto self = instance();
std::unique_lock<std::mutex> lock(self._mutex);
auto it = self._callbacks.find(ident);
if (it != self._callbacks.end()) {
auto my_copy = std::move(it->second);
self._callbacks.erase(it);
lock.unlock();
my_copy();
}
}
private:
std::unordered_map<int, callback_t> _callbacks;
std::mutex _mutex;
int _next_id = 0;
};
now use like so:
// start my timer:
void myclass::start_alien() {
...
_alien_timer_id = timer_dispatch::instance().start_timer(100, std::bind(&myclass::on_alien_timeout, this);
...
}
void myclass::on_alien_timeout() {
// make alien do something, possibly restart a timer...
_alien_timer_id = timer_dispatch::instance().start_timer(100, std::bind(&myclass::on_alien_timeout, this);
}
void myclass::on_alien_killed() {
timer_dispatch::instance().stop_timer(_alien_timer_id);
_alien_timer_id = -1;
}