thoughts about BIC

[Michael Matz]

Hi,

On Sat, 30 Mar 2002, Lubos Lunak wrote:

> On Fri 29. March 2002 15:01, Rolf Magnus wrote:
> > I was wondering if adding member variables always is BIC. Sure, it changes
> > the size of the object (and if you don't append them to the end, the
> > layout), but where is this size needed? AFAIK only on creation of the
> > object, because the system needs to know how much storage to get for it.
> > But if the allocation of storage is only done in the library that contains
> > the class (e.g. for singletons or objects that are only created through a
> > factory), this should not matter, right? So can member variables be added
> > to such a class without making it BIC (if you don't let inline methods
> > access them of course)?

No, it's BIC also in other ways...

>  If you
>
>  - prevent others from creating instances of the class(no public constructor,
> including copy ctor)
>  - prevent others from subclassing it(not even a protected constructor)
>  - prevent others from direct access to any data member(you should have all
> data members at least protected: anyway), including inlines

... if an instance of such a class A is a member of another class B,
extending A will break B by moving it's members just as if you added a
member variable in the middle of B.  Yes, it's a special case of the above
non-instanceability.


Ciao,
Michael.