Pointer Is Fun

What Are Pointers?

Pointers are basically the same as any other variable. However, what is different about them is that instead of containing actual data, they contain a pointer to the memory location where information can be found. This is a very important concept. Many programs and ideas rely on pointers as the basis of their design, linked lists for example.

Getting Started

How do I define a pointer? Well, the same as any other variable, except you add an asterisk before its name. So, for example, the following code creates two pointers, both of which point to an integer:

int* pNumberOne;
int* pNumberTwo;

Notice the "p" prefix in front of the two variable names? This is a convention used to indicate that the variable is a pointer. Now, let's make these pointers actually point to something:

pNumberOne = &some_number;
pNumberTwo = &some_other_number;

The & (ampersand) sign should be read as "the address of" and causes the address in memory of a variable to be returned, instead of the variable itself. So, in this example, pNumberOne is set to equal the address of some_number, so pNumberOne now points to some_number.
Now if we want to refer to the address of some_number, we can use pNumberOne. If we want to refer to the value of some_number from pNumberOne, we would have to say *pNumberOne. The * dereferences the pointer and should be read as "the memory location pointed to by," unless in a declaration, as in the line int *pNumber.

What We've Learned So Far: An Example

Phew! That's a lot to take in. I'd recommend that if you don't understand any of those concepts, to give it another read through. Pointers are a complex subject and it can take a while to master them. Here is an example that demonstrates the ideas discussed above. It is written in C, without the C++ extensions.

#include <stdio.h>

void main()
{
    // declare the variables:
    int nNumber;
    int *pPointer;

    // now, give a value to them:
    nNumber = 15;
    pPointer = &nNumber;

    // print out the value of nNumber:
    printf("nNumber is equal to : %d\n", nNumber);

    // now, alter nNumber through pPointer:
    *pPointer = 25;

    // prove that nNumber has changed as a result of the above by 
    // printing its value again:
    printf("nNumber is equal to : %d\n", nNumber);
}