CODEWITHSUNDEEP
objective-ccocoa
Scott
Scott

Reputation: 1222

NSNumber vs. primitive int for NSArray

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?

Upvotes: 4

Views: 1186

Answers (3)

Aaron Brager
Aaron Brager

Reputation: 66242

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.

Upvotes: 7

zeantsoi
zeantsoi

Reputation: 26193

NSArray deals directly with pointers – and by extension, objects – whereas int is a primitive type. NSNumber exists to bridge this gap. An NSNumber can be cast from an int, float, or even a bool. The salient point is that it provides an object with a pointer, and that pointer can be added to the array.

Upvotes: 1

Jack
Jack

Reputation: 133577

Because Core Foundation classes are not able to work directly with primitive types, they can deal only with NSObject instances (which are objects in OOP terms, not primitives)

That's why you are forced to wrap your int variable with a NSNumber (or NSData or NSValue). This certainly brings some overhead, indeed if you need performance critical code, I suggest you to use Objective-C++ and rely on STL (eg std::vector<int>).

I personally had to discard all core foundation classes in favour of STL for big parts of a game engine just because they couldn't keep it up when managing large amounts of primitive data.

Upvotes: 2

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