Objective-C Fundamentals

60 CHAPTER 3 An introduction to objects
Figure 3.2
variable x
An updated memory map showing how variable y stores the address of
This code snippet declares an integer variable x that’s initialized to the value 45. It
also declares variable y with a data type of int *. The * at the end of the data type indicates
a pointer and means you don’t want to store an actual integer value but rather
the memory address at which one can be found. This pointer is then initialized with
the address of variable x via use of the address-of operator. If variable x had been
stored at address 928 (as previously mentioned), you could graphically represent the
result of executing this code snippet by updating the memory map to be similar to
that shown in figure 3.2.
Notice how the 4 bytes allocated to store variable y now store the number 928.
When interpreted as an address, this indicates the location of variable x, as indicated
by the arrow. The expression y = &x can be read as “place the address of variable x into
variable y.”
3.3.3 Following a pointer
Once you have an address stored in a pointer variable, it’s only natural to want to
determine the value of whatever it points to. This operation is called dereferencing the
pointer and is also achieved by using the * symbol:
int x = 45;
int *y = &x;
NSLog(@"The value was %d", *y);
The statement on the last line prints out the message "The value was 45" because the
* in front of variable y causes the compiler to follow the pointer and access the value it
currently points to. In addition to reading the value, it’s possible to replace it, as demonstrated
next. Confusingly, this operation also makes use of the * operator:
int x = 45;
int *y = &x;
*y = 92;
The statement on the last line stores the value 92 at the address located in variable y.
Referring to figure 3.2, you’ll see that variable y stores (or points to) address 928, so
executing this statement updates the value of variable x even though x is never explicitly
referred to in the statement.