Question

I am just curious with regards to the following, when I am writing code I always try and manage the memory myself by sticking to non-autoreleased objects. I know that this means that objects are not hanging around in the pool, but I was just curious if doing it this way in general is good practice or simply overkill?

// User Managed Memory
NSSet *buttonSizes = [[NSSet alloc] initWithObjects:@"Go", @"Going", @"Gone", nil];
[barItemLeft setPossibleTitles:buttonSizes];
[barItemRight setPossibleTitles:buttonSizes];
[buttonSizes release];

.

// Autoreleased Memory
NSSet *buttonSizes = [NSSet setWithObjects:@"Go", @"Going", @"Gone", nil];
[barItemLeft setPossibleTitles:buttonSizes];
[barItemRight setPossibleTitles:buttonSizes];

Answer 1

Total overkill. If you read the relevant documentation carefully, it does not say that you should avoid autoreleasing objects. It says that you should avoid using autorelease pools when you in tight, object-rich loops. In those cases, you should be managing your memory explicitly (with retain and release) to ensure that objects are created and destroyed in an amortized manner.

The argument that the iPhone is a memory-constrained environment is true, but a complete red herring. Objective-C, with the Foundation and Cocoa frameworks (though they weren’t called that at the time), ran just fine on a NeXTcube, which had 16MB of RAM (expandable to 64). Even the iPhone 3, which is pretty much EOL at this point, has 128MB.

edit

Since an iPhone app is a runloop-based application, a new autorelease pool is going to be created and destroyed every time it spins. This is clearly defined in the documentation. As such, the only reasons you’d have to create your own autorelease pool are:

• Spawning a background thread, where you must create your own pool (since new threads, by default, do not have a base ARPool)
• Performing some operation that will generate a significant number of autoreleased objects. In this case, creating and draining a pool would be to help ensure the limited lifetime of the temporary objects.

However, in the second case, you’re recommended to explicitly manage memory as much as possible. This is so that your operation doesn’t come to a screeching halt when it attempts to drain a pool with a few thousand objects in it. If you manage your memory manually, you can release these objects gradually as they are no longer needed, instead of saving them up for a single all-at-once release. You could help amortize the all-at-once release by nesting ARPools, which will help.

At the end of the day, though, just do what feels natural, and then (and only then) optimize it if you have concrete evidence that you need to do so.

edit #2

OK, it turns out that there is a recommendation to avoid using autorelease. BUT that recommendation is in the “Allocate Memory Wisely” section of the “Tuning for Performance and Responsiveness” area of the iOS Application Programming Guide. In other words, avoid it if you’re measuring a performance problem. But seriously: autorelease has been around for ~20 years and did just fine on computers far slower and more constrained than the iDevices of today.