Re: [__NSFastEnumerationEnumerator nextObject] unexpectedly not very fast!
Re: [__NSFastEnumerationEnumerator nextObject] unexpectedly not very fast!
- Subject: Re: [__NSFastEnumerationEnumerator nextObject] unexpectedly not very fast!
- From: James Bucanek <email@hidden>
- Date: Fri, 25 Sep 2009 08:36:04 -0700
Graham Cox <mailto:email@hidden> wrote (Thursday,
September 24, 2009 6:10 PM +1000):
I thought that NSEnumerator was implemented in terms of fast enumeration
"underneath" - the docs seem to imply that, as does the existence of
__NSFastEnumerationEnumerator private class. It may be still slower than
directly using the fast enumeration syntax, but isn't there some gain to be
had? I have hundreds of these enumeration loops and I still need to support
10.4; I thought I was getting *some* benefit automagically from fast
enumeration on 10.5+
No one replied to your questiona directly, so I'll throw in my
two cents.
Fast enumeration is a 10.5+ObjC2.0 feature only and is
implemented as an alternate method of enumeration. NSEnumerator
in 10.5 is not built on top of fast enumeration or vise versa.
The reason is simple: fast enumeration achieves most of its
speed improvement by fetching objects in batches. The interface
to NSEnumerator fetches one object at a time, so nothing
underneath it is going to avoid the overhead of the per-object
message that it already incurs.
What's mentioned in the documentation is that NSEnumerator
implements the fast enumeration protocol *in order to be
compatible with* fast enumeration syntax. This means that you
can use any NSEnumerator object as the subject of a fast
enumeration statement in Objective-C 2.0:
NSEnumerator *e = [collection enumerator];
for ( id i in e ) ...
But this is not optimized; fast enumeration just calls [e
nextObject] once for each object, so it isn't any faster than
using the NSEnumerator. The only thing you've saved is typing.
In summary, the existence of fast enumeration does nothing for
existing enumeration technologies and if you have to support
10.4 (as I do) you simply can't use it unless you fork your code.
My solution, in the few cases where performance is paramount,
has been to essentially roll my own fast enumeration. For very
large arrays (thousands of objects) I'll get blocks of objects
in batching using [NSArray getObjects:range:], process those in
a tight C loop, and then get another batch.
Also, wouldn't methods such as -makeObjectsPerformSelector:withObject: and
CFArrayApplyFunction(...) gain some benefit from fast enumeration? I use these
a lot too.
I'm sure these kinds of functions are optimized internally and
probably don't use enumerator objects or fast enumeration. But
that's just a guess. The rule is, as always, don't worry about
it unless it's a problem, then profile it, find the bottleneck,
and work around it.
--
James Bucanek
_______________________________________________
Cocoa-dev mailing list (email@hidden)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden