Pointers

A pointer is a variable that stores a memory address, which typically represents the location of another variable. Pointers are useful because they allow the efficient creation and manipulation of complex data structures.

Understanding Pointers

Data is stored in a computer’s memory. A variable represents a specific location within that memory and consists of three parts:

• An identifier (name)
• A value
• An address

The address is a value that describes where in memory a variable is located.

To understand pointers, it may be helpful to think of variables as boxes on a shelf. Each box has a label (the identifier), a unique serial number that allows one to locate it (the address), and possibly something inside of it (the value).

Following this analogy, a pointer can be described as a box that contains the serial number of another box within it.

Declaring and Assigning a Pointer

A pointer is declared similarly to how a variable is — by specifying its type, identifier, and value. However, an asterisk character * is inserted in front of the identifier.

type *name = value;

In order to obtain the address of a variable, the identifier is prefixed with an ampersand symbol &. This is known as the address operator.

int year = 1986;
int *pointer_to_year = &year;

The left-hand side of this declaration may be read in reverse order — “pointer_to_year is a pointer to an integer.”

If the value of year were to be printed, the output would read “1986”. However, if the value of pointer_to_year were to be printed, the output would be an address such as 0x2aba1c0cf890.

The dereference operator (also known as the indirection operator), represented by an asterisk (*), allows one to access the value of the variable that a pointer points to.

int year = 1986;                            // Value of year: 1986

int *pointer_to_year = &year;               // Value of pointer_to_year: 0x2aba1c0cf890

int another_year = *(pointer_to_year) - 33; // Value of another_year: 1953

*(pointer_to_year) = 2019;                  // This sets the value of year to 2019

Functions and Pointers

Like other data types, pointers can be passed to and returned from functions. One scenario in which this might be useful is “returning” two values from a function.

#include <stdio.h>
#include <stdbool.h>

int divide(int a, int b, bool *d) {
  int c = a / b;

  if (c * b == a) {
    *(d) = true;
  } else {
    *(d) = false;
  }
  return c;
}

int numerator = 10;
int denominator = 5;
bool divisible;

int main() {
  int result = divide(numerator, denominator, &divisible);

  printf("%d", result);

  return 0;
}

In the example above, the divide() function returns the result of a / b and sets the variable that d points at to represent whether or not a is divisible by b. When this function is called on the last line, result is assigned a value of 2 and divisible becomes true.