Re: A Data Object in Cocoa
Re: A Data Object in Cocoa
On 10 Jan 2009, at 19:23, Michael Ash wrote:
On Sat, Jan 10, 2009 at 12:39 PM, email@hidden
<email@hidden> wrote:
On 10 Jan 2009, at 17:18, Michael Ash wrote:
On Sat, Jan 10, 2009 at 12:08 PM, Alastair Houghton
<email@hidden> wrote:
On 10 Jan 2009, at 16:48, Michael Ash wrote:
As for underscore being reserved, I have never been able to
figure out
any consequence of a conflict with an Apple ivar name. It may
cause
your source to fail to compile, but it won't cause any *binary*
compatibility problems, which is the real menace.
I *think* the problem here is introspection... if you subclass an
Apple
provided class and add an instance variable with the same name as
one of
theirs, it's possible that any of the dynamic mechanisms the
frameworks
(or,
for that matter, your own code) use to get the value of an object
property
might use the wrong variable, with unexpected results.
Not using an underscore won't save you here. Mechanisms like KVC
will
look for ivars both with and without the underscore. In fact,
leaving
off the underscore makes things worse: with the underscore, the
compiler will at least error when you try to recompile later on,
whereas without it the conflict will exist silently.
A cow-orker just pointed out to me that Apple does not, in fact,
recommend against an underscore prefix for instance variables.
They do
recommend against them for private methods, but that's a completely
different question. It seems that the well-known recommendation
against _ivars is actually a misconception! If I'm wrong and just
missed it, I'd like to know where it says so. But no trace of such a
recommendation in Coding Guidelines for Cocoa: Naming Instance
Variables and Data Types:
http://developer.apple.com/DOCUMENTATION/Cocoa/Conceptual/CodingGuidelines/Articles/NamingIvarsAndTypes.html#/
/apple_ref/doc/uid/20001284
For what it's worth Anguish et al states (p99) that Apple reserves
the right
to change "private" instance variables that begin with an
underscore and no
prefix.
Anguish et al recommend and _ + unique prefix for ivars.
I don't have whichever book you're referring to so I can't see what it
actually says, but your summary makes no sense. Apple can't change
*your* ivars. If it means they reserve the right to change their own
private instance variables that begin with an underscore and no
prefix, well of course, that's what "private" means. That's why you
should never directly twiddle another class's ivars. But that has
nothing to do with how you should name your own ivars.
I agree that it is confusing, or meaningless, but I thought it might
have contributed to the _ misconception.
A Google for - anguish "underscores and usage" - will show the passage
I referred to.
Mike
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Jonathan Mitchell
Central Conscious Unit
http://www.mugginsoft.com
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