Re: Selective Inheritance?

From:
"Daniel T." <daniel_t@earthlink.net>
Newsgroups:
comp.lang.c++
Date:
Tue, 21 Sep 2010 20:45:41 -0400
Message-ID:
<daniel_t-4285C6.20454121092010@70-3-168-216.pools.spcsdns.net>
?? Tiib <ootiib@hot.ee> wrote:

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).


The point is that classes with single member variables should be quite
rare. To have a chain of inheritance from A to X, each with a single
variable is likely silly.

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