Design pattern to avoid downcasting in message passing

Question

Base class MessageHandler has derived classes. They would like to pass messages to each other. Messages could be of different classes, but can be made to share a ba

Answer 1

If I'm understanding your question correctly, you just need straight inheritance with a virtual function. Something like:

class BaseMessage 
{
    public:
    virtual ~BaseMessage() {}

    virtual void processMsg() = 0;
};

class MessageA : public BaseMessage
{
    public:
    MessageA() {}
    virtual ~MessageA() {}    
    virtual void processMsg()
    {
        // ... do something for MessageA ...
    }
};

class MessageB : public BaseMessage
{
    public:
    MessageB() {}
    virtual ~MessageB() {}    
    virtual void processMsg()
    {
        // ... do something for MessageB ...
    }
};

Where you handle the message, simply call the processMsg() function on the message you receive to process each message as specified in each class.

std::auto_ptr m(mailbox.getMessage()); // Returns whatever msg is sent to your handler
m->processMsg();