Nick, I don't know any more than you do about this, except that if
you
search the list archives (cocoabuilder.com), going back five years
or so,
you'll see some long threads with people bitching about how Apple
maybe
"bent the rules" or "violated object-oriented design axioms" with
their
collection classes. I believe the short answer to your first
question is
"because NextStep/Apple says so in their documentation" and a
slightly
longer answer is "because they compromised strict OO design,
probably for
expediency, in designing their collection classes".
The documentation originally quoted makes the issue clear:
"The class cluster architecture involves a trade-off between
simplicity [and as discussed earlier in the article, efficiency] and
extensibility: Having a few public classes stand in for a multitude of
private ones makes it easier to learn and use the classes in a
framework but somewhat harder to create subclasses within any of the
clusters."
etc.
mmalc
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