Question

I am a little curious about the last lines in the two examples presented below (i.e. planetName = [value ??????]) My understanding is that the 1st example with the copy is best as that takes a copy of the string object to protect against the original string object being changed elsewhere.

I am also a little confused by the last line in the 2nd example, again my understanding was that the value object was being passed into the method, I guess I am confused as value is being retained with no associated release? Can someone set me straight?

- (void)setPlanetName:(NSString *)value {
    if (planetName != value) {
        [planetName release];
        planetName = [value copy];
    }
}

.

- (void)setPlanetName:(NSString *)value {
    if (planetName != value) {
        [planetName release];
        planetName = [value retain];
    }
}

Answer 1

Given:

- (void)setPlanetName:(NSString *)value {
    if (planetName != value) {
        [planetName release];
        planetName = [value copy];
    }
}

The -copy ensures that if someone passes in an instance of NSMutableString as value, then planetName won’t change in your instances out from under you.

A good defensive programming pattern. Note that -copy is free on immutable strings; -copy just bumps the retain value and returns self (the instance of the string that was copied).

Now, consider your second example:

- (void)setPlanetName:(NSString *)value {
    if (planetName != value) {
        [planetName release];
        planetName = [value retain];
    }
}

Perfectly valid and works fine, just not as defensive.

In both cases, planetName must be released in your -dealloc method.

- (void) dealloc
{
    [planetName release];
    planetName = nil; // defensive
    [super dealloc];
}

It doesn’t matter if the string is copied, retained, or was originally a constant string that was passed into your setter. If you retain it (or implied retain it through copy), you must release it.

Note that you can think of this as “escape patterns”. Whenever the existing value of planetName escapes your object, you must release it. That can happen when the object is deallocated or when a new value of planetName is set, hence the -release in the setters.

Or, if on Mac OS X, you could turn on garbage collection and be done with it. In any case, you should be using @property & @synthesize to automatically generate the getter/setter pair.