Why do Boost Parameter elected inheritance rather than composition?

Question

I suppose most of the persons on this site will agree that implementation can be outsourced in two ways:

• private inheritance
• composition

Inheritance is most often abused. Notably, public inheritance is often used when another form or inheritance could have been better and in general one should use composition rather than private inheritance.

Of course the usual caveats apply, but I can't think of any time where I really needed inheritance for an implementation problem.

For the Boost Parameter library however, you will notice than they have chosen inheritance over composition for the implementation of the named parameter idiom (for the constructor).

I can only think of the classical EBO (Empty Base Optimization) explanation since there is no virtual methods at play here that I can see.

Does anyone knows better or can redirect me to the discussion ?

Thanks, Matthieu.

Answer 1

This isn't a library I've ever used, so a glance through the documentation you linked to is the only thing I'm basing this answer on. It's entirely possible I'm about to be wrong, but...

They mention constructor delegation as a reason for using a common base class. You're right that composition could address that particular issue just as well. Putting it all in a single type, however, would not work. They want to boil multiple constructor signatures into a single user-written initialization function, and without constructor delegation that requires a second data type. My suspicion is that much of the library had already been written from the point of view of putting everything into the class itself. When they ran into the constructor delegation issue they compromised. Putting it into a base class was probably closer to what they were doing with the previous functionality, where they knew that both interface and implementation aspects of the functionality would be accessible to the class you're working with.

I'm not slamming the library in any way. I highly doubt I could put together a library like this one in any reasonable amount of time. I'm just reading between the lines. You know, speaking from ignorance but pretending I actually know something. :-)