Objective-C Fundamentals

90 CHAPTER 4 Storing data in collections
4.3.2 The NSValue class
NSNumber is a special subclass of NSValue. While NSNumber provides a convenient and
clean interface for boxing and unboxing numeric-based primitive types, NSValue
allows you to box and unbox any C-style value at the expense of having a slightly more
complex interface to program against.
To box the value of an arbitrary C structure or union, you can take advantage of
NSValue’s valueWithBytes:objCType: message. For example, the following code sample
boxes a RentalProperty structure similar to those created in chapter 3:
RentalProperty myRentalProperty =
{270.0f, @"13 Adamson Crescent", TownHouse};
NSValue *value = [NSValue valueWithBytes:&myRentalProperty
objCType:@encode(RentalProperty)];
This code snippet creates a copy of the rental property structure and places it inside
the NSValue object created by the call to valueWithBytes:objCType:. The valueWith-
Bytes argument is the address of the value you want to store in the NSValue instance,
while objCType and the strange @encode statement allow you to tell NSValue what kind
of data you want it to store.
The process of unboxing an NSValue is different but still relatively straightforward.
Rather than returning the unboxed value directly, the getValue message expects you
to pass in a pointer to a variable it should populate with the value it contains, as demonstrated
here:
RentalProperty myRentalProperty;
[value getValue:&myRentalProperty];
On return, getValue: will have populated the myRentalProperty variable with the
value you previously stored. You might be asking yourself, if NSNumber and NSValue
allow you to get around the restriction of only being able to store objects in an array
or dictionary, will a similar technique allow you to store the equivalent of a nil value?
The answer is, yes.
4.3.3 nil vs. NULL vs. NSNull
In this chapter you learned that nil can’t be stored in an array or dictionary and it’s
used to indicate the absence of a value or entry. But what exactly is nil?
With a standard C-style pointer, the special value NULL indicates the absence of a
value. nil represents the very same concept, except applied to a variable that’s
designed to point to an Objective-C object.
Because nil represents the absence of a value, it can’t be stored in an array or dictionary.
But if you really want to store an empty or nil value, you can use a special
class called NSNull. You can conceptualize NSNull as being another kind of boxing,
similar to NSNumber or NSValue but specialized for storing nil.