Objective-C Fundamentals

Pointers and the difference between reference and value types
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Arrays are pointers in disguise
A variable identifier for a C-style array can at some level be thought of as a simple
pointer. This pointer always points to the first element in the array. As an example,
the following is perfectly valid Objective-C source code:
int ages[50];
int *p = ages;
NSLog(@"Age of 10th person is %d", p[9]);
Notice you can assign the array variable to a pointer variable directly without using
the & operator, and you can use the familiar [] syntax to calculate an offset from the
pointer. A statement such as p[9] is another way to express *(p + 9): it’s a shorthand
way to say “add 9 to the pointer’s current value and then dereference it.”
When working with pointers to structure-based data types, a special dereferencing syntax
is available that allows you to deference the pointer and access a specific field in
the structure in a single step, using the -> operator:
struct box *p = ...;
p->width = 20;
The -> operator on the second line dereferences the pointer p and then accesses the
width field in the structure. While following a pointer to read or alter the value it
points at, it’s sometimes helpful to compare two pointers to check if they point to
identical values.
3.3.4 Comparing the values of pointers
When comparing the values of two pointers, it’s important to ensure you’re performing
the intended comparison. Consider the following code snippet:
int data[2] = { 99, 99 };
int *x = &data[0];
int *y = &data[1];
if (x == y) { NSLog(@"The two values are the same"); }
You might expect this code to emit the message "The two values are the same" but it
doesn’t. The statement x == y compares the address of each pointer, and because x
and y both point to different elements in the data array, the statement returns NO.
If you want to determine if the values pointed to by each pointer are identical, you
dereference both pointers:
if (*x == *y) { NSLog(@"The two values are the same"); }
Now that you understand the concept of pointers and how multiple pointers can reference
the same object, you’re ready to communicate with the object. Communicating
with an object enables you to interrogate it for details it stores or to request that
the object perform a specific task using the information and resources at its disposal.