pointers.md

Pointers

Basic Concepts

Declaration

Pointer variables store memory addresses. They can be initialized in the following ways:

int n, m;       // declares two ints

int *p;         // declares a pointer to an int
p = &m;         // points p to m 

int *q = &n;    // declares a pointer to n

int *i, *j, *k; // declares three pointers, all to ints
i = p;          // points i to the same address as p

As shown above, the & symbol can be used to obtain the memory address of variables. Note that &p would return the address where p is stored.

Pointers may also point to functions. The code below shows the declaration and usage of a pointer to a function that accepts an int * pointer and a char, and returns an int.

int (*fn) (int *, char) = &foo;    // points to a function foo
(*fn) (x, y);                      // calls the function

Dereferencing

After initializing a pointer, the * symbol can also be used to dereference it. This allows you to access and modify the variable being pointed at.

int n = 0;
int *p = &n;
*p = 5;         // n is now equal to 5

Structure Members

There are two different ways to access the members of a structure from a pointer. Generally, arrow notation is preferred.

/* Creates a Point structure and a point, pt. */
typedef struct {
    int x, y;
} Point;          

Point pt;

/* Declares a pointer to pt and accesses its values. */
Point *p = &pt;
(*p).x = 0;         // changes x using parentheses
p->y = 1;           // changes y using arrow notation

The NULL Pointer

The NULL pointer is a special pointer with an address of 0. Since 0 is a falsey value, we can check for NULL pointers easily. The if statement below will evaluate to true:

int *p = NULL;  // a null pointer
if (!p) {
    // do stuff
}

Generic Pointers

Generic poiners are declared using void * and can point to data of any type. They should be used sparingly, as they can produce more bugs and introduce security issues.

Arrays

Declaration

Array variables are pointers to the first element of the array. Declaring an array simply reserves a block of memory.

int a[5];             // declares an array of 5 ints
int b[] = {1, 2, 3};  // declares and initializes an array of 3 ints

Pointer Arithmetic

Addition and subtraction can be done with pointers to move through memory. Adding 1 or subtracting 1 to a pointer alters its stored memory address by sizeof(pointer type). In an array, this is equivalent to indexing the next element.