Objective-C Fundamentals

Pointers and the difference between reference and value types
59
You may have noticed that the NSString example contained a * character before the
variable name. This character has special significance (you’ll notice it’s also present in
the declaration of the id data type discussed in the sidebar). This additional character
indicates that the data type is a pointer. But what exactly does this mean?
3.3 Pointers and the difference between reference and value types
A variable in an application consists of four components:
■
■
■
■
Name
Location (where it’s stored in memory)
Type (what kind of data it can store)
Current value
Until now, we haven’t covered where variables are located or whether they are accessible
by means other than their names. Understanding these concepts is closely related
to the concept of pointers.
3.3.1 Memory maps
You can consider the memory of the iPhone as being made up of a large pile of bytes,
each stacked one on top of another. Each byte has a number associated with it, called
an address, similar to houses having an associated street number. Figure 3.1 represents
several bytes of the iPhone’s memory, starting at address 924 and extending through
address 940.
When you allocate a variable in your application, the compiler reserves an explicit
amount of memory for it. For example, a statement such as int x = 45 causes the compiler
to reserve 4 bytes of memory to store the current value of x. This is represented
in figure 3.1 by the 4 bytes starting at address 928.
3.3.2 Obtaining the address of a variable
Referring to the name of a variable in an expression will access or update its current
value. By placing the address-of (&) operator in front of a variable name, you can learn
the address at which the variable is currently stored.
A variable that can store the address of another variable is called a pointer because
it’s said to “point to” the location of another value. The following code snippet demonstrates
how you can use the address-of operator.
int x = 45;
int *y = &x;
Figure 3.1 Representation of a region of the iPhone’s memory showing the location
of variable x