Objective-C Fundamentals

Boxing
89
that doesn’t stop you from shipping it by temporarily placing it inside a box that meets
FedEx’s requirements.
Unfortunately, Objective-C doesn’t provide for the automatic boxing and unboxing
of primitive data types. Therefore, to store an integer or other primitive value in
an array or dictionary, you must perform the boxing and unboxing yourself. This is
the purpose of the NSNumber class.
4.3.1 The NSNumber class
NSNumber is an Objective-C class that can be used to wrap a value of a primitive data
type such as int, char, or BOOL into an object and then allow that value to be extracted
at a later stage. It’s most useful for allowing you to place values of primitive data types
into data structures such as NSArray or NSDictionary that can only store objects.
Manually boxing a primitive value into an NSNumber instance is fairly straightforward:
you call one of NSNumber’s factory methods, such as numberWithInt:
NSNumber *myNumber = [NSNumber numberWithInt:5];
There are similar factory messages called numberWithFloat:, numberWithBool:, and
so on, to allow boxing of other common primitive data types.
Now that the integer value is boxed inside an NSNumber (which is an object), you
can store it in your array, as you originally intended:
[myArray addObject:myNumber];
Boxing the integer inside an NSNumber also means that when you go to fetch the value
from the array, you need to perform the reverse operation to extract the primitive
value from the NSNumber instance. You can do this with a code snippet similar to the
following that uses NSNumber’s intValue message:
NSNumber *myNumber = [myArray objectAtIndex:0];
int i = [myNumber intValue];
The NSNumber class has various other methods that conform to the naming convention
xxxValue, where xxx is replaced with the name of a primitive data type. It’s not
an error to box a value via numberWithInt: and then retrieve it via a method such as
floatValue. Although you stored an integer and fetched a float, this is acceptable. In
this scenario, the NSNumber class performs a typecast operation similar to those discussed
in chapter 2 to convert the value into your desired data type.
A little bit of fancy footwork was required, but in the end, boxing and unboxing
primitive values so you can use them as if they were objects wasn’t too bad. What happens,
though, if you want to store one or more RentalProperty structures in an
NSArray? These structures are also not objects, but it’s doubtful that the NSNumber
class has a numberWithRentalPropertyDetail method available for you to box them.
The answer to this conundrum is another closely related class called NSValue.