Pointers and Memory

Abstract

This document explains how pointers and memory work and how to use them—from the
basic concepts through all the major programming techniques. For each topic there is a
combination of discussion, sample C code, and drawings.

Audience
This document can be used as an introduction to pointers for someone with basic
programming experience. Alternately, it can be used to review and to fill in gaps for
someone with a partial understanding of pointers and memory. Many advanced
programming and debugging problems only make sense with a complete understanding
of pointers and memory — this document tries to provide that understanding. This
document concentrates on explaining how pointers work. For more advanced pointer
applications and practice problems, see the other resources below.


Pace
Like most CS Education Library documents, the coverage here tries to be complete but
fast. The document starts with the basics and advances through all the major topics. The
pace is fairly quick — each basic concept is covered once and usually there is some
example code and a memory drawing. Then the text moves on to the next topic. For more
practice, you can take the time to work through the examples and sample problems. Also,
see the references below for more practice problems.


Topics
Topics include: pointers, local memory, allocation, deallocation, dereference operations,
pointer assignment, deep vs. shallow copies, the ampersand operator (&), bad pointers,
the NULL pointer, value parameters, reference parameters, heap allocation and
deallocation, memory ownership models, and memory leaks. The text focuses on pointers
and memory in compiled languages like C and C++. At the end of each section, there is
some related but optional material, and in particular there are occasional notes on other
languages, such as Java.


Pointers and Memory – document #102 in the Stanford CS Education Library. This and
other free educational materials are available at http://cslibrary.stanford.edu/102/. This
document is free to be used, reproduced, sold, or retransmitted so long as this notice is
clearly reproduced at its beginning.



Basic Pointers

Pointers — Before and After
There's a lot of nice, tidy code you can write without knowing about pointers. But once
you learn to use the power of pointers, you can never go back. There are too many things
that can only be done with pointers. But with increased power comes increased
responsibility. Pointers allow new and more ugly types of bugs, and pointer bugs can
crash in random ways which makes them more difficult to debug. Nonetheless, even with
their problems, pointers are an irresistibly powerful programming construct. (The
following explanation uses the C language syntax where a syntax is required; there is a
discussion of Java at the section.)


Why Have Pointers?
Pointers solve two common software problems. First, pointers allow different sections of
code to share information easily. You can get the same effect by copying information
back and forth, but pointers solve the problem better. Second, pointers enable complex
"linked" data structures like linked lists and binary trees.


What Is A Pointer?
Simple int and float variables operate pretty intuitively. An int variable is like a
box which can store a single int value such as 42. In a drawing, a simple variable is a
box with its current value drawn inside.