Re: Selective Inheritance?

From:
=?ISO-8859-1?Q?=D6=F6_Tiib?= <ootiib@hot.ee>
Newsgroups:
comp.lang.c++
Date:
Tue, 21 Sep 2010 05:02:13 -0700 (PDT)
Message-ID:
<6e2317dc-be7b-4a94-8bb7-3d249052f8b8@a19g2000vbi.googlegroups.com>
On Sep 21, 2:40 pm, "Daniel T." <danie...@earthlink.net> wrote:

Fabrizio J Bonsignore <synto...@gmail.com> wrote:

On Sep 20, 5:35 pm, Suraj <surajprakash...@gmail.com> wrote:

hello,

I need to develop a mechanism to selectively inherit member variables
and functions from Base into Derived.


class Base {int a;};

class A: public Base {int b;};
class B: public C {int c;};
class D: public D {int d;};
...
class Derived-X: public X; //include all members til class X

I also posed myself this problem when I learned C++ and it IS the C++
problem. Most classes show the same concern as to reduce size for it
and its possible derivations. Some solutions can be quite complex and
are not worth the while when you consider pointers vs architecture
complexity. I think that was the reason for virtual inheritance and
templates to make it into the language... but it only makes sense for
functionality bundles, not to bundle together data types, for this
last case it is much better to make as many structs suitably named as
data type combinations are relevant...


One of the prime reasons to make a class is to ensure that an invariant
between two or more variables is never broken, the class becomes the
guardian of the variables so it can restrict operations on them to keep
the invariant. As such, it makes little sense to have a class with only
one variable in it, unless you specifically want to restrict the range
of that variable.


I suspect that you forget number of senses there.

Some that i see to be used in several places:
* encapsulating/wrapping that variable to make it compatible with some
interface.
* hiding information (even type of) that variable.
* restricting transitions between the values of that variable.
* keeping invariant with external single state (variable being id in
some, possibly hidden, storage).

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