William shakespeare hamlet a pointer is the memory

William Shakespeare,
Hamlet
A pointer is the memory address of a variable. Recall from Chapter 5 that the
computer’s memory is divided into numbered memory locations (called bytes )
and that variables are implemented as a sequence of adjacent memory loca-
tions. Recall also that sometimes the C++ system uses these memory addresses
as names for the variables. If a variable is implemented as, say, three memory
locations, then the address of the first of these memory locations is sometimes
10.1

Pointers 405
used as a name for that variable. For example, when the variable is used as a call-by-
reference argument, it is this address, not the identifier name of the variable, that is
passed to the calling function. An address that is used to name a variable in this way (by
giving the address in memory where the variable starts) is called a pointer because the
address can be thought of as “pointing” to the variable. The address “points” to the vari-
able because it identifies the variable by telling where the variable is, rather than telling
what the variable’s name is.
You have already been using pointers in a number of situations. As noted in the pre-
vious paragraph, when a variable is a call-by-reference argument in a function call, the
function is given this argument variable in the form of a pointer to the variable. As
noted in Chapter 5, an array is given to a function (or to anything else, for that matter)
by giving a pointer to the first array element. (At the time we called these pointers
“memory addresses,” but that is the same thing as a pointer.) These are two powerful
uses for pointers, but they are handled automatically by the C++ system. This chapter
shows you how to write programs that directly manipulate pointers rather than relying
on the system to manipulate the pointers for you.
■ POINTER VARIABLES
A pointer can be stored in a variable. However, even though a pointer is a memory
address and a memory address is a number, you cannot store a pointer in a variable of
type
int
or
double
. A variable to hold a pointer must be declared to have a pointer
type. For example, the following declares
p
to be a pointer variable that can hold one
pointer that points to a variable of type
double
:
double *p;
The variable
p
can hold pointers to variables of type
double
, but it cannot normally
contain a pointer to a variable of some other type, such as
int
or
char
. Each variable
type requires a different pointer type.
1
In general, to declare a variable that can hold pointers to other variables of a specific
type, you declare the pointer variable just as you would declare an ordinary variable of
that type, but you place an asterisk in front of the variable name. For example, the fol-
lowing declares the variables
p1
and
p2
so they can hold pointers to variables of type
int
; it also declares two ordinary variables
v1
and
v2
of type
int
:
int *p1, *p2, v1, v2;
There must be an asterisk before each of the pointer variables. If you omit the second
asterisk in the above declaration, then
p2
will not be a pointer variable; it will instead be
an ordinary variable of type
int
.