Question

When an NSString object is passed in as an argument, should I always do retain and release:

-forExample:(NSString*)str{
    [str retain];

    //do something

    [str release];
}

or not? Where and when should I use this?

Answer 1

The reference count of that object isn’t going to change over the course of this method, so there is no reason to send it retain. From Apple’s Memory Management essay (which you should definitely look over):

If you receive an object from elsewhere in your program, it is normally guaranteed to remain valid within the method or function it was received in. If you want it to remain valid beyond that scope, you should retain or copy it. If you try to release an object that has already been deallocated, your program crashes.

You only need to retain an object when you need it to stick around past the current scope.

- (void) forExample: (NSString *)theString {
    // Will need this object later. Stuff it into an ivar and retain it.
    // myString = [theString retain];    
    // For NSString, it's actually better to copy, because this
    // could be a _mutable_ string, and it would in fact be best to use
    // the setter for the ivar, which should deal with the memory management:
    [self setMyString:theString];

    // Do things...

    // Don't release.
}

If you have retained an object, you then need to send it release when you no longer need it.

- (void) otherExample {
    [myString doYourStringThing];
    // If you don't need the string past this point (which would be slightly
    // unusual -- it leaves an ivar unset) release it and set it to nil.
    // [myString release]; myString = nil;
    // Again, it would be best to use the setter:
    [self setMyString:nil];
}

// Generally you keep ivars around until the instance is deallocated,
// and release them in dealloc
- (void) dealloc {
    [myString release];
    [super dealloc];
}