Topics covered on Wednesday, 2010-08-18

Pointers

Variable attributes

• Each variable has 3 attributes associated with it:
  • Its type (e.g. int, char, etc.)
  • Its value (e.g. 10, 'a', etc.)
  • Its address (location in memory).

Getting the Address of a Variable

Given a variable

   var

use the construct

   &var

to get its address. &var is termed a reference to var.

Pointer Variables

A pointer variable is a special variable that can hold the address of another variable. To declare a pointer variable pi which can hold the address of an int variable, use the construct:

   int *pi;

Then you can assign the address of another variable (say var) to pi like:

   pi = &var;

Using a Pointer

You can use a pointer variable by itself, or by dereferencing it.

Example 1: Using a pointer by itself

   int i;
   int *p1;
   int *p2;
   p1 = &i;       // Now p1 contains the address of i
   p2 = p1;       // Now p2 also contains the address of i

As you can see, pointers may also be treated like other variables; they may be assigned to, and their value retrieved.

Example 2: Dereferencing a pointer

   int i = 10;    // i now has the value 10
   int *p1;
   p1 = &i;       // p1 now contains the address of i
   (*p1) = 20;    // Whatever variable p1 had the address of now contains 20
   cout << i << endl;  // Prints 20

Pointer Arithmetic

Adding to and subtracting from a pointer changes the value by the size of the item pointed to. If an int is 4 bytes and pi is a pointer to int, then:

   pi = 200; 

will set the value of pi to 200. Then if you do:

   pi = pi + 1; 

pi will now contain the address of the next int in memory, i.e. 204.

Arrays and Pointers

An array definition stores contiguous space for a number of variables of a given type. E.g. int a[4]; will reserve space for 4 variables of type int, called a[0], a[1], a[2] and a[3].

The array name without a subscript, a is a special case. a by itself refers to the address of the first element of the array, and is of type pointer to int.

In other words,

  a == &a[0]

a can be used like any other pointer to int. However, a is a constant, not a variable.

Further Reading

http://www.cplusplus.com/doc/tutorial/pointers/ contains a reasonable tutorial on pointers in C++.