Do I need to autorelease an object before sending it to NotificationCenter as a userdata?

Question

I need to post a notification using postNotificationName:object:userInfo: method, and I'm passing a custom class FileItem in as userInfo so I can obtain it on the other end. Should I use autorelease like this

FileItem *item = [[[FileItem alloc] init] autorelease];
[[NSNotificationCenter defaultCenter] postNotificationName:@"dataReceived" object:self userInfo:item];
[item release];

or can I just alloc and then release the object immediately after passing it to the default notification center?

FileItem *item = [[FileItem alloc] init];
[[NSNotificationCenter defaultCenter] postNotificationName:@"dataReceived" object:self userInfo:item];
[item release];

I'm trying to get convention here as in can I assume that whenever I pass an object as parameter in a message to another object, the receiving object would do a retain if it needs to, and that I can safely release the said parameter?

Thanks in advance.

Answer 1

The second option is the correct one. You could also just do the following:

FileItem *item = [[[FileItem alloc] init] autorelease];
[[NSNotificationCenter defaultCenter] postNotificationName:@"dataReceived" object:self userInfo:item];

The conventional wisdom is that for every alloc, copy, or retain, you need a corresponding release (or autorelease). Doing anything more is almost guaranteed to result in your object being overreleased.

Answer 2

autorelease just means “send release to this later”. Sending autorelease and then release to the same object is releasing it twice. As Matt Ball says, your latter example and his example are equivalent.

More to the point, you only release what you own. Once you release it, you stop owning it, and should consider it no longer yours. In your first example, after the first release, you've stopped owning that object. The second release is then clearly wrong, because it releases an object you don't own.

And never release an object that some other object owns, unless you also own it.