Re: Newbie Question re Allocation & Initialization

[Simon Wolf]

On 20 May 2008, at 01:18, Brad Gibbs wrote:

On pages 36-7 of Aaron Hillegass' new book, he provides sample code for a Foundation Tool called Lottery. The code is below:

   NSMutableArray *array;
        array = [[NSMutableArray alloc] init];
        int i;
        for (i = 0; i < 10; i++) {
                NSNumber *newNumber = [[NSNumber alloc] initWithInt:(i * 3)];
                [array addObject:newNumber];
        }
        
        for (i = 0; i < 10; i++) {
                NSNumber *numberToPrint = [array objectAtIndex:i];
                NSLog(@"The number at index %d is %@", i, numberToPrint);
        }




        
        for (i = 0; i < 10; i++) {
                NSNumber *numberToPrint;
                numberToPrint = [[NSNumber alloc] init];
                numberToPrint = [array objectAtIndex:i];
                NSLog(@"The number at index %d is %@", i, numberToPrint);
        }
        

He allocates memory for and initializes the first two objects, array and newNumber. But, in the second 'for loop', numberToPrint is neither allocated nor initialized, but the program compiles and runs as expected. I replaced the second 'for loop' with the following, just to see what would happen: It compiled and ran as expected, too. But, when I tried to eliminate allocation and initialization for newNumber in the first 'for loop', the app threw an exception. I don't see an explanation in the book re why numberToPrint can be, but doesn't need to be allocated or initialized. Is it because numberToPrint is simply pointing to newNumber objects in the array that have already been allocated and initialized? Could someone please explain this?

Brad,

From one Cocoa newbie to another, here's my take on it...

newNumber is going to be used to store a number in memory and therefore it needs to allocate or reserve an area of memory via the alloc statement. numberToPrint is simply pointing to the already reserved memory so you do not need to allocate an area of memory again or initialise it a second time. I'd recommend reading the Masters of the Void guide, particularly the section on memory which you can find at http://masters-of-the-void.com/book5.htm

As you discovered, you can allocate memory for numberToPrint but you are simply using up memory to store a value which is already stored somewhere else.

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