I am trying to write networking code and I need to figure out how to structure the different types of messages. I have some bytes coming in from UDP which I want structured as my header and the message content. The header will contain the type of message it is, based on some index (like 5 for a move player packet). I want to be able to define behaviours for individual messages, and be able to treat them as generic messages (myMessage.DoStuff();). How do I get this without making a giant switch statement (or if-elseif)?

You can use a jump table, for example. You could define something similar to:

struct IPacketHandler
{
    virtual void ProcessMessage( Message& message ) = 0;
};
 
class MessageDispatcher
{
    IPacketHandler* m_packetHandlers[ MAXIMUM_HANDLERS ];
 
public:
    void RegisterHandler( IPacketHandler *packetHandler, int messageId )
    {
        assert( messageId >= 0 && messageId < MAXIMUM_HANDLERS );
        m_packetHandlers[ messageId ] = packetHandler;
    }
 
    void Dispatch( Message &message )
    {
        if ( NULL != m_packetHandlers[ message.id ] )
            m_packetHandlers[ message.id ]->ProcessMessage( message );
    }
}

This is not a complete solution, just to demonstrate what I mean. There would need to be more error checking/handling, initialization etc. and it could be expanded on a fair amount also. This is also C++ code, but the equivalent in almost any other language should be pretty similar.

n!

void Dispatch( Message &message )
{ 
      if ( NULL != m_packetHandlers[ message.id ] )
            m_packetHandlers[ message.id ]->ProcessMessage( message );
}

This is extraordinarily dangerous. The message is coming over UDP, it is completely untrusted, and you are not validating the message ID as being within the range of valid message handlers. As an attacker, this makes compromise of the machine almost trivial.

As I said, it's just a basic illustration and needs a lot of checking and error handling added, though no more than any other implementation. There is nothing inherently wrong with the approach suggested itself.

n!

The approach isn't wrong, but it is inherently more dangerous than a switch statement precisely because of the risk of an out of bounds index to a function pointer table.

If you get a switch statement wrong, it will do the wrong thing. If you get a function pointer wrong, it will do an arbitrary thing, and in this case, arbitrary is exactly what an attacker is looking for.

Yes, that's why you need to validate what you're using to index into the table. If you've already validated the input is fine, you can't do anything arbitrary no matter what they've sent you (because you've validated that you can't).

There is nothing inherently more dangerous in the given example over a switch statement, as long as your code has the correct validation there (which the original post already points out).

n!

So here's what I ended up doing:

I used a Dictionary to look up my message types. I store the dictionary as static in the abstract Message class. I also have a static constructor in each child message class which adds it to the dictionary. The dictionary stores event handlers which call a static method in the child message to create an instance of the message. The instance of the message is then stored in the event args and returned through there, for the calling class to call Message.DoStuff().

I suppose I could simplify it by having the event call a static method to actually perform the actions as I am already passing in my header and the data through the event argument.

Once I get something more game like, I will also implement it as a switch statement and see which performs better.