C++ design question - Network packets and serialization

Question

I have, for my game, a Packet class, which represents network packet and consists basically of an array of data, and some pure virtual functions

I would then like to have classes deriving from Packet, for example: StatePacket, PauseRequestPacket, etc. Each one of these sub-classes would implement the virtual functions, Handle(), which would be called by the networking engine when one of these packets is received so that it can do it's job, several get/set functions which would read and set fields in the array of data.

So I have two problems:

1. The (abstract) Packet class would need to be copyable and assignable, but without slicing, keeping all the fields of the derived class. It may even be possible that the derived class will have no extra fields, only function, which would work with the array on the base class. How can I achieve that?
2. When serializing, I would give each sub-class an unique numeric ID, and then write it to the stream before the sub-class' own serialization. But for unserialization, how would I map the read ID to the appropriate sub-class to instanciate it?

If anyone want's any clarifications, just ask.

-- Thank you

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Edit: I'm not quite happy with it, but that's what I managed:

Packet.h: http://pastebin.com/f512e52f1
Packet.cpp: http://pastebin.com/f5d535d19
PacketFactory.h: http://pastebin.com/f29b7d637
PacketFactory.cpp: http://pastebin.com/f689edd9b
PacketAcknowledge.h: http://pastebin.com/f50f13d6f
PacketAcknowledge.cpp: http://pastebin.com/f62d34eef

If someone has the time to look at it and suggest any improvements, I'd be thankful.

Answer 1

You need to look up the Factory Pattern.

The factory looks at the incomming data and created an object of the correct class for you.

Answer 2

Yes, I'm aware of the factory pattern, but how would I code it to construct each class? A giant switch statement? That would also duplicade the ID for each class (once in the factory and one in the serializator), which I'd like to avoid.

Answer 3

For copying you need to write a clone function, since a constructor cannot be virtual:

virtual Packet * clone() const = 0;

Which each Packet implementation implement like this:

virtual Packet * clone() const {
    return new StatePacket(*this);
}

for example for StatePacket. Packet classes should be immutable. Once a packet is received, its data can either be copied out, or thrown away. So a assignment operator is not required. Make the assignment operator private and don't define it, which will effectively forbid assigning packages.

For de-serialization, you use the factory pattern: create a class which creates the right message type given the message id. For this, you can either use a switch statement over the known message IDs, or a map like this:

struct MessageFactory {
    std::map<Packet::IdType, Packet (*)()> map;

    MessageFactory() {
        map[StatePacket::Id] = &StatePacket::createInstance;
        // ... all other
    }

    Packet * createInstance(Packet::IdType id) {
        return map[id](); 
    }
} globalMessageFactory;

Indeed, you should add check like whether the id is really known and such stuff. That's only the rough idea.

Answer 4

To have a Factory class that does not know about all the types ahead of time you need to provide a singleton where each class registers itself. I always get the syntax for defining static members of a template class wrong, so do not just cut&paste this:

class Packet { ... };

typedef Packet* (*packet_creator)();

class Factory {
public:
  bool add_type(int id, packet_creator) {
    map_[id] = packet_creator; return true;
  }
};

template<typename T>
class register_with_factory {
public:
  static Packet * create() { return new T; }
  static bool registered;
};

template<typename T>
bool register_with_factory<T>::registered = Factory::add_type(T::id(), create);

class MyPacket : private register_with_factory<MyPacket>, public Packet {
//... your stuff here...

  static int id() { return /* some number that you decide */; }
};

Answer 5

Why do we, myself included, always make such simple problems so complicated?

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Perhaps I'm off base here. But I have to wonder: Is this really the best design for your needs?

By and large, function-only inheritance can be better achieved through function/method pointers, or aggregation/delegation and the passing around of data objects, than through polymorphism.

Polymorphism is a very powerful and useful tool. But it's only one of many tools available to us.

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It looks like each subclass of Packet will need its own Marshalling and Unmarshalling code. Perhaps inheriting Packet's Marshalling/Unmarshalling code? Perhaps extending it? All on top of handle() and whatever else is required.

That's a lot of code.

While substantially more kludgey, it might be shorter & faster to implement Packet's data as a struct/union attribute of the Packet class.

Marshalling and Unmarshalling would then be centralized.

Depending on your architecture, it could be as simple as write(&data). Assuming there are no big/little-endian issues between your client/server systems, and no padding issues. (E.g. sizeof(data) is the same on both systems.)

Write(&data)/read(&data) is a bug-prone technique. But it's often a very fast way to write the first draft. Later on, when time permits, you can replace it with individual per-attribute type-based Marshalling/Unmarshalling code.

Also: I've taken to storing data that's being sent/received as a struct. You can bitwise copy a struct with operator=(), which at times has been VERY helpful! Though perhaps not so much in this case.

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Ultimately, you are going to have a switch statement somewhere on that subclass-id type. The factory technique (which is quite powerful and useful in its own right) does this switch for you, looking up the necessary clone() or copy() method/object.

OR you could do it yourself in Packet. You could just use something as simple as:

( getHandlerPointer( id ) ) ( this )

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Another advantage to an approach this kludgey (function pointers), aside from the rapid development time, is that you don't need to constantly allocate and delete a new object for each packet. You can re-use a single packet object over and over again. Or a vector of packets if you wanted to queue them. (Mind you, I'd clear the Packet object before invoking read() again! Just to be safe...)

Depending on your game's network traffic density, allocation/deallocation could get expensive. Then again, premature optimization is the root of all evil. And you could always just roll your own new/delete operators. (Yet more coding overhead...)

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What you lose (with function pointers) is the clean segregation of each packet type. Specifically the ability to add new packet types without altering pre-existing code/files.

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Example code:

class Packet
{
public:
  enum PACKET_TYPES
  {
    STATE_PACKET = 0,
    PAUSE_REQUEST_PACKET,

    MAXIMUM_PACKET_TYPES,
    FIRST_PACKET_TYPE = STATE_PACKET
  };

  typedef  bool ( * HandlerType ) ( const Packet & );

protected:
        /* Note: Initialize handlers to NULL when declared! */
  static HandlerType  handlers [ MAXIMUM_PACKET_TYPES ];

  static HandlerType  getHandler( int thePacketType )
    {   // My own assert macro...
      UASSERT( thePacketType, >=, FIRST_PACKET_TYPE    );
      UASSERT( thePacketType, <,  MAXIMUM_PACKET_TYPES );
      UASSERT( handlers [ thePacketType ], !=, HandlerType(NULL) );
      return handlers [ thePacketType ];
    }

protected:
   struct Data
   {
            // Common data to all packets.
     int  number;
     int  type;

     union
     {
       struct
       {
         int foo;
       } statePacket;

       struct
       {
         int bar;
       } pauseRequestPacket;

     } u;

   } data;


public:

  //...
  bool readFromSocket() { /*read(&data); */ }  // Unmarshal
  bool writeToSocket()  { /*write(&data);*/ }  // Marshal

  bool handle() { return ( getHandler( data.type ) ) ( * this ); }

}; /* class Packet */

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PS: You might dig around with google and grab down cdecl/c++decl. They are very useful programs. Especially when playing around with function pointers.

E.g.:

c++decl> declare foo as function(int) returning pointer to function returning void
void (*foo(int ))()
c++decl> explain void (* getHandler( int  ))( const int & );
declare getHandler as function (int) returning pointer to function (reference to const int) returning void