NSNumber vs. primitive int for NSArray

Question

I am learning Objective-C/Cocoa using the 4th edition of Cocoa Programming for Mac OSX. I apologize for the basic question but I'm one of those people that really need to understand the insides of everything in order for it to make sense to me and this book doesn't always do that to my needs. I've already picked up the basics of C++ and now I'm learning Obj-C so that is my background. This is a snippet of code that I'm learning...

for (i= 0; i < 10; i++) {
    NSNumber *newNumber = [[NSNumber alloc] initWithInt:(i * 3)];
    [array addObject:newNumber];
}

My question is why would you create instances of NSNumber to add to the array instead of just a single integer variable. It seems like it is just extra overhead creating a new instance each loop that could be avoided. But I'm sure the authors know what they are doing and there is a reason for it, could someone please explain?

Answer 1

NSArray can only hold objects, not primitive types. NSNumber and NSValue are used to wrap primitive types in objects, so they can be stored in foundation collections like NSArray, NSDictionary, and NSSet (and their mutable / ordered counterparts). As Peter Hosey points out in the comments:

The common term for this is "boxing" (as in boxing them up), and the inverse — extracting the primitive value from a box object — is unboxing.