Re: Is Apple's singleton sample code correct?
Re: Is Apple's singleton sample code correct?
- Subject: Re: Is Apple's singleton sample code correct?
- From: David Gimeno Gost <email@hidden>
- Date: Sat, 26 Nov 2005 18:21:43 +0100
On 26 Nov 2005, at 14:28, Uli Kusterer wrote:
+ (id) allocWithZone: (NSZone*) zone
{
@synchronized( self ) {
if ( sharedInstance == nil ) {
sharedInstance = [super allocWithZone: zone];
} else {
[sharedInstance retain];
}
}
return sharedInstance;
}
Errm ... good start, but you meant to be @synchronized() on some other
object. If you synchronize to self, two threads create a "self", and
both sync to their own "self"s, so you effectively get the same result
as if you didn't have the @synchronized call in there at all. You need
an explicit NSLock here, or one shared object that you know you can
rely on already existing.
Well, I didn't actually mean anything to be @synchronized(). I just
copied that part of the Apple's sample code and didn't think about it.
If you think there is a bug there, I suggest you file a bug report :-).
In addition, you *also* need to retain the sharedInstance you get back
from allocWithZone. You need to retain once for the reference
sharedInstance keeps, plus once more for the caller who called
allocWithZone: and expects to get an object they need to release.
Yes, you're right, and this is indeed easily fixed, but I think the
+sharedInstance method should also be modified accordingly, because in
that case the shared instance variable and the instance variable the
result of the allocation is assigned to are the same:
+ (id) allocWithZone: (NSZone*) zone
{
@synchronized( self ) {
if ( sharedGizmoManager == nil ) {
sharedGizmoManager = [super allocWithZone: zone];
}
}
// The +sharedInstance method assumes that an extra -retain message
is
// being sent here when the singleton is first allocated, just in
case the
// result of this method is assigned to another instance variable.
// Be sure to revise the +sharedInstance method if this policy
changes.
return [sharedGizmoManager retain];
}
+ (MyGizmoClass*) sharedInstance
{
@synchronized( self ) {
if ( sharedGizmoManager == nil ) {
// The extra -retain in +allocWithZone: is needed only when
the
// result of [[alloc] init] is assigned to a variable other
than
// our static shared instance, so we undo it here.
sharedGizmoManager = [[[self alloc] init] release];
}
}
return sharedGizmoManager;
}
Regards.
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