Date: Wed, 21 May 2008 08:33:40 +0800
From: "Michael Ash" <[EMAIL PROTECTED]>
On Tue, May 20, 2008 at 4:07 PM, Peter Duniho <[EMAIL PROTECTED]>
wrote:
But personally, it makes me nervous to have a language that allows
the
implementation of a class to change according to other code not
related to
the class. It's one thing if you're just adding a method that you
want to
use somewhere else. But in Objective-C you can also add a method
that
overrides a base class method, or would not normally exist in the
class but
which has meaning to some other code that might check for it.
I'd just like to point out that the above applies to any OO language
which supports both subclassing and dynamic dispatch, which is to say
approximately all of them. Categories are certainly differently
powerful in this regard, and as such will get you into trouble in ways
that subclassing cannot, but subclassing already has this powerful and
dangerous semantic that you're worried about.
No, it doesn't.
Each language varies a bit, of course. But in the other popular
languages that I know reasonably well -- C++, C#, Java -- a subclass
does not have the ability to touch any part of the base
implementation that is not specifically exposed by that
implementation. Private members are invisible, and other members are
only overridable when the base class allows it by making them virtual.
There is, of course, hazard any time the base implementation makes
this allowance. But those choices can be made on a member-by-member
basis, and reliable decisions and assumptions can be made regarding
the relative dependability of the base class on the basis of what
that base class exposes to sub-classes. Many classes have _no_
overridable members and no "protected" members, and thus completely
encapsulate their implementations. This is, in fact, the norm for
these other languages.
Cocoa restrains class extension _much_ less than any of these other
languages, and in turn has a _much_ higher degree of hazard.
It's certainly a continuum. I agree we're not talking either/or,
black/white, etc. here. Other languages aren't without their danger
zones either. It just seems to me that Cocoa goes overboard, for
relatively little payoff.
YMMV.
Pete
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