How to find type of pointer

user_350048 0
Viewed 90

I have a optional property event that I define as a pointer in this struct:

struct AnimatedSprite {
    //...some other properties
    Event *event;
};

Let's say I want to store a StartTransitionEvent , defined like this:

class Event {
public:
    Event() = default;
};

class StartTransitionEvent: public Event {
public:
    int test = 1;
};

I can create and assign the StartTransitionEvent:

auto sprite = AnimatedSprite{};
auto newEvent = StartTransitionEvent{};
sprite.event = &newEvent;

But how do I get the type once I need to call something like:

eventBus->EmitEvent<StartTransitionEvent>(sprite->event);

Should I do some sort of checking on my event pointer to get to the StartTransitionEvent? Or maybe not use pointers at all and go for a std::variant to store the event with all it's possible child-classes?

c++
2 Answers

You could use double dispatch, a.k.a. visitor pattern and let the specific Event visit the EventBus.

So something like

class Event
{
public:
    virtual void Emit(EventBus& eventBus) = 0;
};

class StartTransitionEvent : public Event
{
public:
    void Emit(EventBus& eventBus) override
    {
        eventBus.EmitEvent(this);
    }
};

sprite.event.Emit(eventBus);
user_16858140

Assuming you're designing such a hierarchy, your EventBus class should be able to work with the base Event class. You achieve that through virtual functions in the Event class:

struct Event {
  Event() = default;
  virtual ~Event() = default;

  virtual void action1(); // event-specific actions
  virtual void action2(); 
  virtual void action3(); 
};

struct StartTransitionEvent : Event {
  void action3() override; // override some actions 
};

struct EventBus {
  void emit(Event& event) {
    // ...
    event.action1();
    // ...
    event.action2();
    // ...
    event.action3();
  }
};

int main() {
  StartTransitionEvent event;
  EventBus bus;
  bus.emit(event);
}
user_101473990
Related