For the most part with ARC (Automatic Reference Counting), we don't need to think about memory management at all with Objective-C objects. It is not permitted to create NSAutoreleasePool
s anymore, however there is a new syntax:
@autoreleasepool {
…
}
My question is, why would I ever need this when I'm not supposed to be manually releasing/autoreleasing ?
EDIT: To sum up what I got out of all the anwers and comments succinctly:
New Syntax:
@autoreleasepool { … }
is new syntax for
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
…
[pool drain];
More importantly:
- ARC uses
autorelease
as well asrelease
. - It needs an auto release pool in place to do so.
- ARC doesn't create the auto release pool for you. However:
- The main thread of every Cocoa app already has an autorelease pool in it.
- There are two occasions when you might want to make use of
@autoreleasepool
:- When you are in a secondary thread and there is no auto release pool, you must make your own to prevent leaks, such as
myRunLoop(…) { @autoreleasepool { … } return success; }
. - When you wish to create a more local pool, as @mattjgalloway has shown in his answer.
- When you are in a secondary thread and there is no auto release pool, you must make your own to prevent leaks, such as
Best Answer
ARC doesn't get rid of retains, releases and autoreleases, it just adds in the required ones for you. So there are still calls to retain, there are still calls to release, there are still calls to autorelease and there are still auto release pools.
One of the other changes they made with the new Clang 3.0 compiler and ARC is that they replaced
NSAutoReleasePool
with the@autoreleasepool
compiler directive.NSAutoReleasePool
was always a bit of a special "object" anyway and they made it so that the syntax of using one is not confused with an object so that it's generally a bit more simple.So basically, you need
@autoreleasepool
because there are still auto release pools to worry about. You just don't need to worry about adding inautorelease
calls.An example of using an auto release pool:
A hugely contrived example, sure, but if you didn't have the
@autoreleasepool
inside the outerfor
-loop then you'd be releasing 100000000 objects later on rather than 10000 each time round the outerfor
-loop.Update: Also see this answer - https://stackoverflow.com/a/7950636/1068248 - for why
@autoreleasepool
is nothing to do with ARC.Update: I took a look into the internals of what's going on here and wrote it up on my blog. If you take a look there then you will see exactly what ARC is doing and how the new style
@autoreleasepool
and how it introduces a scope is used by the compiler to infer information about what retains, releases & autoreleases are required.