Re: A situation where private inheritance is useful
On Jul 16, 3:43 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
Juha Nieminen wrote:
Let me share with you a realization I had about private
inheritance.
As we all know, C++ supports three types of inheritance:
Public, private and protected. From these only the public
inheritance is a "true" inheritance from an object-oriented
point of view. If you inherit privately, there is no "is-a"
relationship between the derived and the base classes, and
thus private inheritance does not conform to object-oriented
design. So why have this odd private inheritance at all?
[...]
As soon as one realizes that inheritance is just what it is,
and any access specifier simply limits the area where the
relationship between the derived class and the base class is,
well, *accessible*, then everything with protected inheritance
(and private inheritance as well) becomes kind of clear. I
probably wouldn't call it an epiphany, but...
So, the "is-a" relationship *does in fact exist* in all three
cases, only it's "known" /to everybody/ in case of the public
inheritance, /only to descendants, members, and friends/ in
case of protected one, and /only to friends and members/ of
the derived class in case of private inheritance.
Simple as that.
It depends on what you mean by the "is-a" relationship. The
expression "is-a" is usually used to refer to the view client
code has of the object, so it would require public inheritance.
On the other hand, I'd disagree that private inheritance doesn't
conform to object-oriented design. Object-oriented includes a
lot of things. The "traditional" explination of private and
public inheritance is inheritance of implementation and
inheritance of interface. I'd consider both OO, although the
LSP only applies to inhertance of interface.
But IMHO, it can be even more complicated than that. (And this
probably relates to what you said.) Consider a GUI class that
implements the callback interface for GUI events. As far as
clients of the class are concerned, this is an implementation
detail---they don't what to know anything about how the class
handles things like mouse events. So it makes perfect sense for
the inheritance from the callback interface to be private, even
though it's inheritance of an interface.
For a good discussion of use of private and public interfaces,
I'd suggest Barton and Nackman. They make use of both
extensively, using private inheritance systematically whenever
it is inheritance of implementation.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34