should variable be released or not? iphone-sdk

Question

Hi, I have the following piece of code from a book.

There is this function loadPrefs where the NSString *userTimeZone is being released before the end of the function.

Why? The string was not created with alloc and I assume that the stringForKey function returns an autoreleased NSString. Is this an error or am I missing something? Is it an error in the book? (I new into objective-C)

In the documentation for stringForKey the only thing it mentions is:

Special Considerations

The returned string is immutable, even if the value you originally set was a mutable string.

The code:

- (void) loadPrefs {
    timeZoneName = DefaultTimeZonePref;

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSString *userTimeZone = [defaults stringForKey: TimeZonePrefKey];
    if (userTimeZone != NULL)
        timeZoneName = userTimeZone;
    [userTimeZone release];
    show24Hour = [defaults boolForKey:TwentyFourHourPrefKey];
}

Thanks!!!!

Answer 1

You're right. There are two things wrong with this code: it's releasing the string from stringForKey: improperly, and it's not retaining userTimeZone when it assigns the value to an instance variable.

Here's a better attempt:

- (void) loadPrefs {
    [timeZoneName release]; // In case there was a previous value

    timeZoneName = DefaultTimeZonePref; // ASSUMPTION: DefaultTimeZonePref is a constant
    // And thus doesn't need retain/release.

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSString *userTimeZone = [defaults stringForKey: TimeZonePrefKey];
    if (userTimeZone != NULL) {
        timeZoneName = [userTimeZone retain];
    }

    show24Hour = [defaults boolForKey:TwentyFourHourPrefKey];
}

And don't forget to release timeZoneName in dealloc.