I'm new to Objective-C and cocoa. In the guide provided by Apple for Cocoa, there is a confusing example in memory management:
Suppose you want to implement a method to reset the counter. You have a couple of choices. The first implementation creates the NSNumber instance with alloc, so you balance that with a release.
- (void)reset {
NSNumber *zero = [[NSNumber alloc] initWithInteger:0];
[self setCount:zero];
[zero release];
}
The second uses a convenience constructor to create a new NSNumber object. There is therefore no need for retain or release messages
- (void)reset {
NSNumber *zero = [NSNumber numberWithInteger:0];
[self setCount:zero];
}
I am not sure why the object is created with 'new' instead of 'alloc & init' does not need to be retained/released. My understanding is that both are doing the same thing, except that with 'alloc & init' we can use custom checks and initialisation.
Many thanks.
The second example returns an auto released object. The code for the convenience constructor probably looks like this, at least when it comes to replicate the functionality.
Autorelease is a way to defer sending a release method to an object while not delegating ownership of the object to the caller of the constructor. This is an important concept, because the naming conventions require you to not return ownership of an object unless your method starts with
copy
new
alloc
orretain
. However, since you can't return an owned object, you would have to callrelease
on it in your convenience constructor, which would then lead to you returning a deallocated object. So, autorelease allows you to return an un-owned object to the caller which will receive an actual release method later (when the current autorelease pool gets drained).Autorelease methods are collected in so called autorelease pools, which are thread local quasi linked lists (they are not implemented as linked lists, but they do work like they were) and that just collect autoreleased objects. Objects can be added to them multiple times by calling, once for each
autorelease
method they receive. When the pools is drained or destroyed, all objects it contains will receive arelease
message. By default, the system will provide you with an auto release pool, at least on the main thread, but you can create your own ones using this code (which is also used in every main method, if you take a look):