Re: private and protected inheritance

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From: Doug Harrison [MVP] (dsh_at_mvps.org)
Date: 11/06/04 

Date: Sat, 06 Nov 2004 11:59:16 -0600

Carl Daniel [VC++ MVP] wrote:

>private inheritance is used to prevent code outside the class from
>performing a derived-to-base pointer conversion. Private inheritance is
>functionally equivalent to embedding an instance of the base class as a
>private member, and many experts recommend using embedding instead of
>private inheritance. The differences are purely syntactic.

There is one significant difference. Derived classes can override virtual
functions belonging to private bases. So in that sense, a private base is
not a mere implementation detail. I used to like private inheritance a lot
for designing classes that think of themselves as "ISA" WRT to their private
bases, but which provide a restricted interface to the outside world. In
particular, using-declarations are quicker to write than forwarding
functions. I still think there's some validity to that, but the overriding
issue, well, overrides it, and embedding should be used for would-be private
bases that have virtual functions.

In some cases, you don't know much about the base, and you might want to
take advantage of the empty-base optimization where possible without running
afoul of any virtual functions the base class has; Nathan Meyers wrote about
that here, which ISTR is where I first heard about the private base
overriding issue:

The "Empty Member" C++ Optimization
http://www.cantrip.org/emptyopt.html 

-- 
Doug Harrison
Microsoft MVP - Visual C++

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