I am trying to develop an event-based state machine for a couple hours now and I'm failing to identify why the templated classes bellow dont match. Basically, State
publishes events and a StateMachine
listens to them. Here goes a snippet:
#include <iostream>
#include <vector>
// Interface for event listening
template <typename Event, typename Sender>
class EventListener
{
public:
virtual ~EventListener() = default;
virtual void onEvent(const Sender* sender, const Event& data) = 0;
};
// Abstract class for event publishing
template <typename Event>
class EventPublisher
{
public:
typedef EventListener<Event, EventPublisher> Listener;
virtual ~EventPublisher() = default;
// Queues an event listener
void attach(Listener* listener) {
listeners.push_back(listener);
}
protected:
// Publishes an event among all registered listeners
void publish(const Event& e) {
for (Listener* listener : listeners) {
listener->onEvent(this, e);
}
}
private:
std::vector<Listener*> listeners;
};
// Concrete publisher & listener
class StateEvent {};
class StateEventPublisher : public EventPublisher<StateEvent> {};
class StateEventListener : public EventListener<StateEvent, StateEventPublisher> {};
class State : public StateEventPublisher {
public:
void foo() {
publish(StateEvent());
}
};
class StateMachine final : public StateEventListener {
private:
void onEvent(const StateEventPublisher* sender, const StateEvent& e) override {}
};
int main()
{
State state;
StateMachine machine; // Is a StateEventListener, which is a EventListener<StateEvent, StateEventPublisher>, whereas StateEventPublisher is a EventPublisher<StateEvent>
state.attach(&machine); // Incompatible with EventListener<StateEvent, EventPublisher<StateEvent>>*
state.foo();
return 0;
}
Why does the machine cant be attached to the state, if a machine is a StateEventListener
, which is a EventListener<StateEvent, StateEventPublisher>
, whereas a StateEventPublisher
is a EventPublisher<StateEvent>
?! What am I doing wrong?!
Thanks a lot!
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