Choosing the right subclass to instantiate programmatically

Question

Ok, the context is some serialization / deserialization code that will parse a byte stream into an 'object' representation that's easier to work with (and vice-versa).

Here's a simplified example with a base message class and then depending on a 'type' header, some more data/function are present and we must choose the right subclass to instantiate:

class BaseMessage {
public:
    enum Type {
        MyMessageA = 0x5a,
        MyMessageB = 0xa5,
    };

    BaseMessage(Type type) : mType(type) { }
    virtual ~BaseMessage() { }

    Type type() const { return mType; } 

protected:
    Type mType;

    virtual void parse(void *data, size_t len);
};

class MyMessageA {
public:
    MyMessageA() : BaseMessage(MyMessageA) { }

    /* message A specific stuf ... */

protected:
    virtual void parse(void *data, size_t len);
};

class MyMessageB {
public:
    MyMessageB() : BaseMessage(MyMessageB) { }

    /* message B specific stuf ... */

protected:
    virtual void parse(void *data, size_t len);
};

In a real examples, there would be hundreds of different message types and possibly several level or hierarchy because some messages share fields/functions with each other.

Now, to parse a byte string, I'm doing something like:

BaseMessage *msg = NULL;
Type type = (Type)data[0];

switch (type) {
    case MyMessageA:
        msg = new MyMessageA();
        break;

    case MyMessageB:
        msg = new MyMessageB();
        break;

    default:
        /* protocol error */
}

if (msg)
    msg->parse(data, len);

But I don't find this huge switch very elegant, and I have the information about which message has which 'type value' twice (once in the constructor, one in this switch) It's also quite long ...

I'm looking for a better way that would just be better ... How to improve this?

Answer 1

One way of approaching it would be using a map and register some kind of factory function for each message type. This means that you get rid of the switch case and can add and remove messages dynamically.

The code would look something like:

// Create the map (most likely a member in a different class)
std::map<BaseMessage::Type, MessageCreator*> messageMap;
...

// Register some message types
// Note that you can add and remove messages at runtime here
messageMap[BaseMessage::MyMessageA] = new MessageCreatorT<BaseMessageA>();
messageMap[BaseMessage::MyMessageB] = new MessageCreatorT<BaseMessageB>();
...

// Handle a message
std::map<Type, MessageCreator*>::const_iterator it = messageMap.find(msgType);
if(it == messageMap.end()) {
    // Unknown message type
    beepHang();
}
// Now create the message
BaseMessage* msg = it->second.createMessage(data);

The MessageCreator class would look something like this:

class MessageCreator {
    public:
    virtual BaseMessage* createMessage(void* data, size_t len) const = 0;
};
template<class T> class MessageCreatorT : public MessageCreator {
    public:
    BaseMessage* createMessage(void* data, size_t len) const {
        T* newMessage = new T();
        newMessage.parse(data, len);
        return newMessage;
    }
};