Pointers

Pointers

• A pointer is a datatype.

• You can create variables of this type as you

would of other types.

• Contains a memory address.

• Points to a specific data type.

int *pointer1 = &x;

• A pointer has to be declared just like any other variable -

remember a pointer is just a variable that stores an

address. For example,

int *p;

• is a pointer to an integer. Adding an asterisk in front of a

variable's name declares it to be a pointer to the

declared type. Notice that the asterisk applies only to

the single variable name that it is in front of, so:

int *p , q;

declares a pointer to an int and an int variable, not two

pointers.

The meaning of two new operators: &

and *

• The & operator returns the address of a

variable

For example:

int *p , q;

declares p, a pointer to int, and q an int and

the instruction: p=&q;

stores the address of q in p

Operator *

• If place * in front of a pointer variable then the result is

the value stored in the variable pointed at.

• That is, p stores the address, or pointer, to another

variable and *p is the value stored in the variable that p

points at.

• The * operator is called the de-referencing operator and

it helps not to confuse it with multiplication

• To declare a pointer add an * in front of its

name.

• To obtain the address of a variable use & in

front of its name.

• To obtain the value of a variable use * in

front of a pointer's name.

Now see if you can work out what the following

means:

int *a , b , c;

b = 10;

a = &b;

c = *a;

• Firstly three variables are declared - a (a pointer to

int), and b and c (both standard integers).

• The instruction is a=&b which stores the address of b

in a

• Finally c = *a stores the value in the variable pointed

to by a in c. As a points to b, its value i.e. 1O is stored

in c

More on Pointers

• You can print the address stored in a

pointer using the %p conversion specifier

Example: printf(“%p”, numPtr);

scanf() needs to know where to put the

value - it needs the address of the variable

as it takes pointers as parameters.

Example: int i;

scanf(“%d”, &i);

Why Pointers?

• To modify a variable in a function that is not a

global, or a local to that function

• To save space

• To save time

• To use dynamic memory

• Used extensively in linked structures

Pointers and Functions

• To enable a function to access and change an

object.

• For large structures it is more efficient.

• Use const specifier whenever a constant is

intended.

Pointers and Arrays

• The name array is equivalent to

&array[0]

• pPtr++ increments pPtr to point to the

next element of array.

• pPtr += n increments pPtr to point to n

elements beyond where it currently points.

• pPtr-qPtr equals i-j.

Pointers and Arrays (cont)

• array[0] is equivalent to *array

• array[n] is equivalent to *(array + n)