Solving the NSInteger <-> NSNumber problem

Question

I've written a large social networking iPhone application, and one of the biggest issues I run into is the fact that NSInteger (and all the other NS-non-object types) are not first class citizens. This problem stems from the fact that, obviously, they have no representation for a nil value.

This creates two main problems:

1. Tons of overhead and opaqueness to convert to and from NSNumber when storing/retrieving from a collection.
2. Can't represent nil. Oftentimes, I want to be able to represent an "unset" value.

One way to solve this is to use NSNumber all the time, but that gets extremely confusing. In a User model object, I would have about 20 different NSNumbers, and no easy way to tell if each one is a float, integer, bool, etc.

So here are my thoughts for potential solutions and the pros/cons. I'm not really sold on any of them, so I thought I'd ask for feedback and/or alternative solutions to this problem.

1. Continue to use NSInteger types, and just use NSIntegerMax to represent nil.
   PRO - Less memory overhead
   PRO - Clear typing
   CON - NSIntegerMax is not really nil. If programmers aren't careful or don't know this convention, invalid values could leak into the display layer.
   CON - Can't store them in a collection without conversions in and out
   
   
2. Use NSNumber and designate types using hungarian notation (eg NSNumber fHeight, NSNumber iAge)
   PRO - First-class citizens
   PRO - Nil problem solved
   CON - Increased memory overhead
   CON - Lose compiler type checking
   CON - Hungarian notation is contentious
   
   
3. Write my own first-class primitive object types (think Java http://developer.android.com/reference/java/lang/Integer.html)
   PRO - First-class citizens
   PRO - Nil problem solved
   PRO - Keeps compiler type checking
   PRO - Objects will be simpler than NSNumber. Internal storage will specific to data type.
   CON - Increased memory overhead
   CON - Sacrifices a bit of code portability and compatibility
   

Looking for a convincing argument in favor of one of these techniques, or one I haven't thought of if you've got one.

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UPDATE

I've gone ahead and started an open source project (Apache 2.0), into which I'll be pulling a number of our internal classes as I have time. It currently includes object wrappers for some of the more common native data types (BOOL, CGFloat, NSInteger, NSUInteger). We chose to do this because it upgrades these data types to first class citizens with strict typing. Maybe you disagree with this approach, but it has worked well for us, so feel free to use it if you want.

I'm adding other classes we've found uses for, including a disk-backed LRU cache, a "Pair" object, a low memory release pool, etc.

Enjoy github - Zoosk/ZSFoundation

Answer 1

The most common convention for representing the idea of nil as an NSInteger is to use the NSNotFound value. This is, in fact, equal to NSIntegerMax, though it tends to be more obvious to the reader that this is a sentinel value representing the lack of a number. There are many cases where this is used throughout Cocoa. One common case is as the location field of an NSRange as a return value from -rangeOfString: et al.

Answer 2

You could try this?

#define numberWithFloat(float f) \
  [NSNumber numberWithFloat:f]
#define floatFromNumber(NSNumber *n) \
  [n floatValue]

(see my original answer below)

Here's the other thing with NSNumber, you don't have to retrieve what you set.

For example

NSNumber *myInt = [NSNumber numberWithInteger:100];
float myFloat = [myInt floatValue];

is perfectly valid. NSNumber's strength is that it allows you to "weak-type" your primitives, use compare:, use isEqualTo:, and stringValue for easy display.

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[EDIT]

User @Dave DeLong says that sub-classing NSNumber is a Bad Idea without much work. Since it's a class cluster (meaning NSNumber is an abstract superclass of a lot of subclasses) you'll have to declare your own storage if you sub-class it. Not recommended, and thanks to Dave for pointing that out.

Answer 3

As an alternative, there is also the NSDecimal struct for representing numerical types. NSDecimal (and its Objective-C class version NSDecimalNumber) lets you represent true decimals and avoid floating point errors, so it tends to be recommended for dealing with things like currency.

The NSDecimal struct can represent numbers as well as a Not a Number state (a potential nil replacement). You can query whether an NSDecimal is not a number using NSDecimalIsNotANumber(), and generate the Not a Number value with the help of an NSDecimalNumber.

NSDecimals are faster to work with than NSDecimalNumbers, and the structs don't bring the same kind of memory management issues that objects do.

However, there isn't an easy way to get values into NSDecimal form without using a temporary NSDecimalNumber. Also, many of the math functions (like trigonometry operations) that are available for simple floating point numbers aren't yet available for NSDecimal. I'd like to write my own functions that add some of these capabilities, but they would require accessing the internal fields of the struct. Apple labels these as private (with the use of an underscore), but they are present in the headers and I'd guess they're unlikely to change in the future.

Update: Dave DeLong wrote a series of functions for performing NSDecimal trigonometry, roots, and other operations. I've tweaked these to provide up to 34 digits of decimal precision, and the code for these functions can be downloaded here. As a warning, this precision comes at a price in terms of performance, but these functions should be fine for fairly infrequent calculations.

Answer 4

I don't understand why, if you store stuff as an NSNumber, you need to care what type it is. NSNumber will choose an appropriate internal representation and then do any necessary conversion depending on what format you ask for its value in.

If you can't tell what type is appropriate from the name of the variable you need to choose better names. e.g.

numberOfChildren = [NSNumber numberWithFloat: 2.5];

is plainly silly.

And for currency values, I definitely recommend using NSDecimalNumber (which is a subclass of NSNumber) but defines base 10 arithmetic operations so you don't have to worry about float rounding.