Question

Consider this example:

NSMutableArray *aArray;
[Util doSomeMagic:aArray];
[aArray count];

- (void)doSomeMagic:(NSMutableArray *)aArray
{
    if(aArray == NULL)
    {
      aArray = [[NSMutableArray alloc] init];
    }
    [aArray addObject:@"magic"];
    [aArray addObject:@"magic2"];
}

The doSomeMagic: method will fill in the array. Inside doSomeMagic:, I can get the array count correctly, but outside the method, the array remains empty. How can I let the doSomeMagic: method change the content that is passed in?

Answer 1

Your doSomeMagic: method needs to receive the array by reference, ensuring it modifies the object it receives:

- (void)doSomeMagic:(NSMutableArray *)array
{
    // for example, let's remove the last element in the array
    [array removeLastObject];
}

I’m guessing your doSomeMagic: might look like this (which doesn’t work because it modifies a copy of the array for scope):

- (void)doSomeMagic:(NSMutableArray *)array
{
    NSMutableArray *newArray = [NSMutableArray arrayWithArray:array];
    // for example, let's remove the last element in the array
    [newArray removeLastObject];
}

EDIT: I see you’ve added your doSomeMagic: code. You need to understand pass by reference here. Since you call doSomeMagic: with a nil array (because you declared aArray without setting it to point to anything afterwards), when doSomeMagic: runs it assigns a new array to nil, and then the method returns with no visible effect. I suggest changing the code where you call it to:

NSMutableArray *aArray = [NSMutableArray array];
[Util doSomeMagic:aArray];
[aArray count];

and your doSomeMagic: method to:

- (void)doSomeMagic:(NSMutableArray *)aArray
{
    [aArray addObject:@"magic"];
    [aArray addObject:@"magic2"];
}

Keep in mind that since I am using the class array on NSMutableArray, the array returned will be autoreleased after [aArray count];. The final code would look like this:

NSMutableArray *aArray = [[NSMutableArray array] retain];
[Util doSomeMagic:aArray];
[aArray count];
[aArray release];