Bart Lateur <[EMAIL PROTECTED]> writes:
> The problem is that you can't safely subclass a class, without examining
> it's source, just to make sure that your instance fields don't clash
> with any private fields of the mother class.
Well...let's think twice. What has the reputatin of being a bug or a
shortcoming in Perl is actually a feature in CLOS.
In the C++ View of the World you should be able to derive from a class
without examining the bases - especially the indirect bases. I thought
it made sense for years.
Until I realized that the 'miraculous reuse by accident' is a complete
lure. Which did a lot of harm to the reputation of OO. I now believe
that reuse is the consequence of ad-hoc design and cooperation between
the involved classes, that it has to be planned for and that it has a
cost - typically two or three times the cost of a non-reusable
design. Someone even said that the word 'reuse' made it look too
simple and suggested using the word 'polybloodyhardreuse' instead <g>
Thus, regardless of the issue about field inheritance, I believe that
"you can't safely subclass a class, without examining it's source" (or
its documentation, which should spell out the existence and purpose of
each field).
Also, consider what happens in presence of multiple inheritance,
particularly in the 'mixin' style. Typically you want to share a
single base sub-object even if a class is inherited more than once
(iow you want what C++ calls 'virtual inheritance'). Currently, in
Perl you get that by default. If you introduce scoped fields (by
default), you'll also have to introduce two kinds of inheritance:
shared and repeated.
I'm more in favor of a mechanism that makes it easy to build field
names from the package name, for those rare cases where you want
scoped fields. There were discussions about this a couple of years ago
on p5p. For example:
package Foo;
sub new
{
bless { "${CURRENT_PACKAGE}name" => 'Simpson' ...
}
...where $CURRENT_PACKAGE is a special variable automatically set
to...guess what? ;-)
--
Jean-Louis Leroy
http://users.skynet.be/jll
