Objective-C Fundamentals

50 CHAPTER 2 Data types, variables, and constants
HOW DO ARRAYS DIFFER FROM SIMPLE TYPES?
Arrays behave a little bit differently from the data types we’ve discussed up to this
point. For example, you may expect the following code snippet to print the result
A=1, B=2, C=3 because of the assignment statement array2 = array1 setting the second
array to the contents of the first:
int array1[3] = { 1, 2, 3 };
int array2[3] = { 4, 5, 6 };
array2 = array1;
NSLog(@"A=%d, B=%d, C=%d", array2[0], array2[1], array2[2]);
But if you attempt to build this code sample, you should notice a compiler error complaining
cryptically about “incompatible types in assignment.” What this error is
attempting to convey is that the variable array2 can’t be used on the left-hand side of
the assignment (=) operator: it can’t be assigned a new value, at least not in the manner
shown here.
In chapter 3, as we discuss the object-oriented features of Objective-C, we explore
concepts such as pointers and the difference between value and reference types,
which will help explain why Objective-C refuses to accept what on initial glance looks
like a perfectly acceptable request.
2.4.4 The importance of descriptive names
Objective-C provides a way to declare an alternative name for an existing data type via
a statement known as a type definition, or typedef for short. Typedefs can be useful
when the built-in data type names aren’t descriptive enough or if you want to differentiate
the purpose or intent of a variable from its physical data type (which you may
decide to change at some point in time).
You’ve already seen typedefs, although they weren’t pointed out as such: the NSInteger
and NSUInteger data types discussed earlier are typedefs that map back to the
built-in datatypes int and long int as required. This isn’t Apple-specific magic, simply
a couple of lines of prespecified source code that automatically get included in every
Cocoa Touch application.
To declare your own typedefs, you use the typedef keyword followed by the name
of an existing data type (or even the specification of an entirely new one) followed by
the new name you want to associate with it.
As an example, you could assign the alternative name cube to the struct box data
type you previously declared by adding the following type definition:
typedef struct box cube;
Or you could merge the declaration of struct box into the typedef statement:
typedef struct box {
int width;
int height;
int depth;
} cube;