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

EDIT: Ooopss! I posted the answer below because I misread your post. I thought you said the Boost library used composition over inheritance, not the other way around. Still, if its usefull for anyone... (See EDIT2 for what I think could be the answer for you question.)

I don't know the specific answer for the Boost Parameter Library. However, I can say that this is usually a better choice. The reason is because whenever you have the option to implement a relationship in more than one way, you should choose the weakest one (low coupling/high cohesion). Since inheritance is stronger than composition...

Notice that sometimes using private inhertiance can make it harder to implement exception-safe code too. Take operator==, for example. Using composition you can create a temporary and do the assignment with commit/rollback logic (assuming a correct construction of the object). But if you use inheritance, you'll probably do something like Base::operator==(obj) inside the operator== of the derived class. If that Base::operator==(obj) call throws, you risk your guarantees.

EDIT 2: Now, trying to answer what you really asked. This is what I could understand from the link you provided. Since I don't know all details of the library, please correct me if I'm wrong.

When you use composition for "implemented in terms of" you need one level of indirection for the delegation.

struct AImpl 
{ 
  //Dummy code, just for the example.
  int get_int() const { return 10; }
};

struct A
{
AImpl * impl_;
int get_int() const { return impl->get_int(); }
/* ... */
};

In the case of the parameter-enabled constructor, you need to create an implementation class but you should still be able to use the "wrapper" class in a transparent way. This means that in the example from the link you mentioned, it's desired that you can manipulate myclass just like you would manipulate myclass_impl. This can only be done via inheritance. (Notice that in the example the inheritance is public, since it's the default for struct.)

I assume myclass_impl is supposed to be the "real" class, the one with the data, behavior, etc. Then, if you had a method like get_int() in it and if you didn't use inheritance you would be forced to write a get_int() wrapper in myclass just like I did above.

Answer 2

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. :-)