Question

I am just reading through the Practical Memory Management guide.

I am somewhat confused by this block of code:

- (void)printHello {
    NSString *string;
    string = [NSString stringWithFormat:@"Hello"];
    NSLog(@"%@", string);
}

It seems to me that string is going to have a reference count of 0. Is this true?

What stops string from being deallocated before we call NSLog(string)?

Is this somehow equivalent to this:

- (void)printHello {
    NSString *string;
    string = [[[NSString stringWithFormat:@"Hello"] retain] autorelease];
    NSLog(@"%@", string);
}

---

Edit: Similarly this code is given in the Practical Memory Management
guide:

- (NSString *)fullName {
    NSString *string = [NSString stringWithFormat:@"%@ %@", firstName, lastName];
    return string;
}

When and how does the return value get freed? Who is the owner? Does the caller of fullName need to release the string returned by full name?

Answer 1

Strictly speaking,

- (void)printHello {
    NSString *string;
    string = [NSString stringWithFormat:@"Hello"];
    NSLog(@"%@", string);
}

Is not equivalent to

- (void)printHello {
    NSString *string;
    string = [[[NSString stringWithFormat:@"Hello"] retain] autorelease];
    NSLog(@"%@", string);
}

The convention is that a method should autorelease any object it returns. The only exception (AFAIK) is for constructors, which return an object with a +1 retain count. Since [NSString stringWithFormat:] returns an object. In first snippet, stringWithFormat: returns an already autoreleased object. the second snippet, you’re retaining it once more and it’ll be released twice (which has the same effect, but the second retain/autorelease pair is redundant).

Ok, now to answer your question. Essentially, every time UIKit calls your code, it creates an NSAutoreleasePool object. Every time you autorelease an object, its added to this pool. Finally, when your code returns back to UIKit, it calls the drain method on the pool (i.e [pool drain]) and that releases every object which has been added to the pool and deallocates the pool. Also, autorelease pools can be nested, so you can create your own pools and drain them if you’re going to be creating a lot of autoreleased objects. It isn’t as complicated as it sounds.

I’d highly recommend that you read the Autorelease Pools chapter in the Memory Management Guide (Which incidentally, comes right after the Practical Memory Management chapter).