Question

Apparently mutableCopy copies by reference, not value. Ie if I do this:

NSMutableArray arrayA = [arrayB mutableCopy];

then change values of arrayB, then arrayA’s values will also be changed.

I think Java has a clone() method to copy by value.. is there an equivalent in objective c?

Answer 1

The mutableCopy method performs “shallow” copy. Each element of arrayA is a reference to an object that is also in arrayB. If you add elements to arrayA (or remove elements), arrayB will be unchanged, and vice versa. But since the elements of arrayA and arrayB reference the same objects, a change to one of those objects “shows up” in both arrays.

If you want a one-level deep copy of arrayB, you can do this:

NSMutableArray *arrayA = [[NSMutableArray alloc] initWithArray:arrayB copyItems:YES];

That will have this effect:

NSMutableArray *arrayA = [[NSMutableArray alloc] init];
for (id element in arrayB) {
    [arrayA addObject:[element copy]]; //copies immutable objects to new array  
}