Objective C memory management

Question

The following code is on apple's website.

1) Inside the setMyArray method, is it necessary to release myArray before setting a new value to it? I thought setting the value of an object to a new object, will release the old object from memory.

2) why does it say myArray = [newArray mutableCopy];, instead of simply saying myArray = newArray;? Is it necessary to pass a mutable copy to our property?

@interface MyClass : NSObject {
    NSMutableArray *myArray;
}
@property (nonatomic, copy) NSMutableArray *myArray;
@end

@implementation MyClass
@synthesize myArray;

- (void)setMyArray:(NSMutableArray *)newArray {
    if (myArray != newArray) {
        [myArray release];
        myArray = [newArray mutableCopy];
    }
}
@end

EDIT: Would it be the same if myArray was (nonatomic, retian)

Apple Documentation
copy
    Specifies that a copy of the object should be used for assignment. (The default is assign.)
    **The previous value is sent a release message**.

Answer 1

With respect to #1, yes you must release myArray before setting a new value to it, otherwise you'll leak. Note the property, it's labelled as a copy, meaning that myArray will hold an object with a reference count of one once its set.

And with regards to #2, because if you don't mutableCopy, you're not getting a copy of the object, you're simply pointing at the other thing. So, if the other thing goes away, you'll have a pointer to a dangling object. Bad things ensue from that point forward.

Answer 2

1 - In order to avoid a memory leak you must release the array before setting a new value to it.

2 - mutableCopy is required in the context you provided in order to geta copy of the object, and not just a pointer. If the original array goes away, you'd be left with a dirty pointer.

FYI, copies in Objective-C are shallow, not deep copies. This means that when you do [array copy] or [array mutableCopy], you get an array which is separate from the original array, but which still points to all the same objects. If you want to perform a deeper copy, you'll have to do it manually, for example by iterating through the original array, making copies of the contents, and adding those copies to a new array.