Comments on: Cocoa memory management for smarties, Part 3: accessors

By: Jeff

Jeff — Wed, 08 Oct 2008 16:09:05 +0000

Dave, it's true that both approaches would have the same result. The equality check is a minor optimization, because pointer comparison is faster than Objective-C messages. In most cases, though, it probably doesn't matter. If performance is an issue, there are various ways of optimizing the setter.

Scott, it should be clear from my post that the accessors were never intended to be thread-safe.

By: Scott

Scott — Mon, 06 Oct 2008 17:45:11 +0000

I have read at some point that in a multithreaded app it is possible that one thread can actually return a released object if it hits at the wrong time:
For example in your setter….

if (_thing != aThing)
{
[_thing release];
// if your getter hit here on a different thread — you have problems.
_thing = [aThing retain];
}

A safer approach is to release the old object AFTER the the pointer to the new object has been assigned. use a local pointer to keep the reference of the ivar so it can be released afterwards

-(void) setThing:(id)aThing{
if (_thing!= aThing){
id oldThing = _thing;
_thing = [aThing retain];
[oldThing release];
}
}

By: Dave

Dave — Mon, 06 Oct 2008 14:04:55 +0000

This code:
-(void) setThing:(id)aThing
{
if (_thing != aThing)
{
[_thing release];
_thing = [aThing retain];
}
}

Can be easily rewritten as:
-(void) setThing:(id)aThing {
[aThing retain];
[_thing release];
_thing = aThing;
}

By: Jeff

Jeff — Mon, 06 Oct 2008 05:01:57 +0000

Michal, thanks!

Andrew, that's a good question. Suppose that on line 2 you have [updater updateThings] instead of [myThing setThing:nil], where updateThings contains the following code:

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
...
[[allThings objectAtIndex:i] setThing:nil];
...
[pool release];

Since the setter's [_thing autorelease] occurs within the context of this nested pool, _thing could still be deallocated before line 3.

By: Andrew

Andrew — Sun, 05 Oct 2008 21:13:17 +0000

Admittedly I’m still a bit of a Cocoa beginner, but why wouldn’t changing [_thing release] to [_thing autorelease] in the setter suffice?

By: Michal

Michal — Sun, 05 Oct 2008 18:05:43 +0000

Never read such funny and still true intro to cocoa’s memory management. And I’ve read a bunch of them since I started with objective-c in middle 90s.