Re: Solving the data inheritance problem

From:

"Greg Herlihy" <greghe@pacbell.net>

Newsgroups:

comp.lang.c++.moderated

Date:

6 Dec 2006 01:13:29 -0500

Message-ID:

<1165344702.869584.71780@73g2000cwn.googlegroups.com>

Kaba wrote:

Hello, I'd like to discuss about solutions to the following general
problem:

Given is an abstract class A (that is, contains at least one virtual
function). The derived classes of A are called B[i] (indexed by i).

The main purpose of inheritance is interface inheritance: this is to say
that all B[i] have commonly describable behaviour.

I think most C++ experts would disagree. Inheritance (or creating a
subtype) is about substitutability - B is an A and can be used
wherever an A is needed. So inheritance is best thought of in terms of
behavior (or more formally, a contract) than in terms of interfaces and
code re-use. See the Liskov Substitution Principle"
http://en.wikipedia.org/wiki/Liskov_Substitution_Principle and "C++
Coding Standards" by Sutter and Alexandrescu (especially #32 and
following).

Now assume that the B[i] also have data in common: every B[i] will
contain the same block of data which is accessed using a fixed set of
functions.

If I now make the data handling functions virtual functions of A, I will
end up implementing identical boiler-plate code for each B[i]
implementation. This naturally raises objection: code replication should
be avoided.

The question now is: how do you model common data between B[i] without
replicating code?
Any ideas for better solutions?

If B wants A's data and to reuse its code than A should be made a
member of B. Of course, "A" and "B" are a little vague as concepts, so
it's a little hard to create a model for them that would fit any actual
A or B.

Greg

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