How to avoid downcast?

问题

I have an implementation of a State Pattern where each state handles events it gets from a event queue. Base State class therefore has a pure virtual method void handleEvent(const Event*). Events inherit base Event class but each event contains its data that can be of a different type (e.g. int, string...or whatever). handleEvent has to determine the runtime type of the received event and then perform downcast in order to extract event data. Events are dynamically created and stored in a queue (so upcasting takes place here...).

I know that downcasting is a sign of a bad design but is it possible to avoid it in this case? I am thinking of Visitor Pattern where base class State would contain virtual handlers for each event but then again downcast will need to take place in the piece of code which dequeues event from a queue and passes it to the current state. (At least in this case big switch(eventID) would be only at one place...). Is Visitor Pattern the best way (best practice) to avoid downcasting?

Here is the pseudo-code (I am passing boost::shared_ptr in this example but downcasting happens anyway):

enum EventID
{
   EVENT_1,
   EVENT_2,
   ...
};

class Event
{
   EventID id;
public:
   Event(EventID id):id(id){}
   EventID id() const {return id;}
   virtual ~Event() = 0;
};

class Event1 : public Event
{
   int n;
public:
   Event1(int n):Event(EVENT_1), n(n){}
   int getN() const {return n;}
};

class Event2 : public Event
{
   std::string s;
public:
   Event2(std::string s):Event(EVENT_2), s(s){}
   std::string getS() const {return s;}
};

typedef boost::shared_ptr<Event> EventPtr;

class State
{
   ...
public:
   ...
   virtual ~State() = 0;
   virtual void handleEvent(const EventPtr& pEvent) = 0;
};

class StateA : public State
{
   ...
public:
   void handleEvent(const EventPtr& pEvent)
   {
      switch(pEvent->id())
      {
         case EVENT_1:        
            int n = boost::static_pointer_cast<Event1>(pEvent)->getN();
            ...
            break;
         case EVENT_2:
            std::string s = boost::static_pointer_cast<Event2>(pEvent)->getS();
            ...
            break;
         ... 

      }
   }   
}

回答1:

The typical visitor pattern performs no downcast, thanks to a double-dispatch strategy:

// Visitor.hpp
class EventBar;
class EventFoo;

class Visitor {
public:
    virtual void handle(EventBar const&) = 0;
    virtual void handle(EventFoo const&) = 0;
};

// Event.hpp
class Visitor;

class Event {
public:
    virtual void accept(Visitor&) const = 0;
};

And the implementations:

// EventBar.hpp
#include <Event.hpp>

class EventBar: public Event {
public:
    virtual void accept(Visitor& v);
};

// EventBar.cpp
#include <EventBar.hpp>
#include <Visitor.hpp>

void EventBar::accept(Visitor& v) {
    v.handle(*this);
}

The key point here is that in v.handle(*this) the static type of *this is EventBar const&, which selects the correct virtual void handle(EventBar const&) = 0 overload in Visitor.

回答2:

The idea of events is to pass detailed objects through generalized (and agnostic) interface. Downcast is inevitable and part of the design. Bad or good, it's disputable.

Visitor pattern only hides the casting away from you. It's still performed behind the scenes, types resolved via virtual method address.

Because your Event already has the id, it's not completely agnostic of the type, so casting is perfectly safe. Here you're watching the type personally, in visitor pattern you're making compiler take care of that.

"Whatever goes up must go down".

来源：https://stackoverflow.com/questions/10718771/how-to-avoid-downcast