Question

Completely new to Objective-C, trying to find out when I need to alloc and release objects.

For example, I want to fetch some data from the Web. I found an article at Apple which has this code:

NSURLRequest *theRequest=[NSURLRequest requestWithURL:
                  [NSURL URLWithString:@"http://www.apple.com/"]
                    cachePolicy:NSURLRequestUseProtocolCachePolicy
                timeoutInterval:60.0];
// create the connection with the request
// and start loading the data
NSURLConnection *theConnection=[[NSURLConnection alloc] 
                                 initWithRequest:theRequest delegate:self];

What I don’t understand is: Why do they need to call alloc on the connection but not on the request? How do I know when I need alloc and when not?

Similar questions for release. From what I read, I only ever need to release objects that were initialized using alloc/init. But all “initWithXXX” functions return autoreleased objects instead.

Is this a hard rule, or just convention? Is there a way to find out if I need to release an object or not?

Answer 1

Please read the Memory Management Programming Guide. All of these are explained there. Also check out Learn Objective-C.

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Why do they need to call alloc on the connection but not on the request?

Whenever you need to own an object to make it live longer than the function’s scope, the object needs to be -retained. Of course you could use

NSURLConnection* theConnection = [NSURLConnection connectionWithRequest:theRequest
                                                               delegate:self];

but the connection will be -autoreleased. But because there’s time for a connection to finish, this variable should be -retained to prevent the connection becoming invalid. Of course, then, you could do

NSURLConnection* theConnection = [[NSURLConnection connectionWithRequest:theRequest
                                                               delegate:self] retain];

But this is equivalent to +alloc → -init → -autorelease → -retain, the last two steps are redundant. Probably that’s why Apple chooses to use +alloc/-init here.

(BTW, this style would cause the static analyzer to complain. It’s better to store theConnection as an ivar somewhere, e.g. in the delegate object.)

On the other hand, the NSURLRequest is just a temporary object, so it needs to be -released when the function ends. Again, you could use

NSURLRequest* theRequest = [[NSURLRequest alloc] initWithURL:...];
...
[theRequest release];

this is even more efficient, as the autorelease pool won’t be filled up, but using this method one may forget to -release and cause a leak.

But all “initWithXXX” functions return autoreleased objects instead.

No, -init… should never return an -autoreleased object.