I'm playing with SDL, and I am trying to supply a function pointer to an event filter. This works fine if I make the function a static member of ObjectWithState
, but I'd like to have the callback function alter the state of the object. I was hoping to do this perhaps using a functor, but I can't quite work it out.
Is there any C++11 trickery that I can use to make this work?
class ObjectWithState
{
int someState;
public:
int operator()(void* userData, SDL_Event *event)
{
return ++someState;
}
};
int main()
{
//boilerplate
ObjectWithState obj;
SDL_EventFilter f = &(obj.operator()); //ERROR -> cannot create non-constant pointer to member function
SDL_SetEventFilter( f, nullptr );
}
Use the userdata
parameter to point to your object, and dispatch through a static method to the non-static method:
class ObjectWithState
{
int someState;
public:
int operator()(SDL_Event *event)
{
++someState
}
static int dispatch(void* userdata, SDL_Event* event)
{
return static_cast<ObjectWithState*>(userdata)->operator()(event);
}
};
int main()
{
//boilerplate
ObjectWithState obj;
SDL_SetEventFilter(&ObjectWithState::dispatch, &obj);
}
You can't assign pointer to member functions to C style function pointers. You have to use a free function or a static function, and then call whatever members you need inside that.
Actually, std::bind
may allow you to do it. Not entirely sure though.
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