I’ve got 2 applications (lets call them AppA and AppB) communicating with each other.
AppA is sending objects to AppB.
There could be different objects and AppB does not support every object.
An object could be a Model (think of a game, where models are vehicles, houses, persons etc).
There could be different AppBs. Each supporting another base of objects.
E.g. there could be an AppB which just supports vehicle-models. Another AppB just supports specific airplane-models.
The current case is the following:
There is a BasicModel which has a position and an orientation.
If another user wants extra attributes, he inherits an ExpandedModel. And adds e.g. an attribute Color.
Now every user who needs additional attributes inherits from a more general model. After a while there is a VehicleModel which could activate windshield-wipers, an AircraftModel which could have landing lights or a PersonModel which could wave goodbye when a certain boolean is set to true.
The AppB always needs to be customized if it should support a new Model.
This approach has a big disadvantage: It’s getting extremely complex after a few inheritations. Perhaps there are redundancies like an ExpandedAircraftModel which could use windschield-wipers too.
Another approach:
I create just one Model-class which has an attribute-list. The most easy implementation would be a std::map where the Key is the attribute-name and the Value is the attribute-value.
The user could now enter as much information as he wants. If he wants to use a windshieldwiper he just adds a "windshieldwiper - ON"-pair.
If AppB supports windshieldwipers it just looks if there is such an attribute in the list and reads the related value.
A developer of AppB needs to document well what attributes he supports. Every developer has to check if the specific attribute is already existing and how it is called (e.g. one developer could name his attribute windshieldwiper and another calls it windshield-wiper)
This could get extremely complex too and the only thing a user can relate to is the documentation or a specific standard-specification which has to be kept at a central space.
Finally, the question:
Which approach is better?
Did you see any additional disadvantages?
Is there a third approach which should be used instead of these two?
Just for a comparison, Google’s Protocol Buffers uses a combination of both but leans hard toward your second example.
If you have distinctly different data that needs to be sent over the channel, you use the tool to generate a derivitive of the “message” class, but each message can contain other messages, and you can nest message definitions in themselves. When a message is sent out, the receiver checks the fields to determine what type of message it is and what fields are contained within.
The downside is that your code becomes overly verbose very quickly, as you can’t really use inheritance to automate the process of acting on an incoming message, but the upside is that your protocol messages stay highly organized and easy to DEBUG since you’re using a reflexive attribute list of sorts.