Class 08: Pointers

Held Wednesday, February 9, 2000

Overview

Today we consider one of the most important features of C, pointers. Pointers provide power to your programs, but inappropriate use of points can destroy programs.

Notes

• If you want more information on pointers and memory, it might help to read the Stanford CS Library's documentation on pointers and memory. I've made a local copy.
• As some of you have learned (or already know), I get much too much email. This means that I tend to leave less crucial email until later. In particular, I typically ignore emailed assignments until I grade them. Hence, if you send in a question on an assignment the same day an assignment is due, I will not read it until much later. Please start messages you want me to read with something like "READ ME NOW!" or "IMPORTANT" or "HELP".
• Are there questions on homework 2?

Contents

• Pointers
  • Behind the Scenes
  • Pointers in Java
  • Pointers in C

Summary

• Variables
• Pointers and addresses
• Pointers in various langauges
• Pointer pitfalls

Pointers

• By this point in your careers, you should be fairly comfortable with the notion of variables.
• From my perspective, variables are containers that store values.
• What kinds of values can they store? At the most basic level, they can store values of the primitive types, such as int.
• In some languages, variables can store the address of another container. We call such addresses pointers
• Why is it useful to store addresses?
  • It provides a form of generality. If we have many containers that we want to change, we can put each address in turn into the same container and then affect them indirectly.
  • It lets us dynamically allocate memory. You can't (easily) implement dynamic data structures without some form of pointer.
• We often draw pictures, but we may also describe pointers numerically.

Behind the Scenes

• In order to understand pointers, it may help to look ``behind the scenes'' in the implementation of a typical programming language.
• In practice, the containers we talked about before are just areas of memory.
• A symbol table maps every variable name (and parameter) to a corresponding address in memory.

Pointers in Java

• In Java, every variable that contains objects is really a pointer variable that points to the object.
• That's why we have all the fun aliasing problems.

Pointers in C

• In C, you must explicitly indicate that something is a pointer.
• Given a type, t, you declare a variable that points to containers of type t with t *.
• For example, here's how you say ``ip points to an integer'':

  int *ip;
  

• What can you do with pointers?
  • You can copy them with the assignment operator, =

    int *ip
    int *jp;
    ...
    ip = jp;
    

  • You can determine the contents of the container they point to with *

    printf("%d\n", *ip);
    

  • You can change those contents, too

    *ip = 2;
    

  • You can assign the address of another container to them. You get the address of a variable with &

    ip = &i;
    

  • You can allocate new memory and get a pointer to it with malloc.

    ip = (int *) malloc(sizeof(int));
    

  • You can pass them as parameters:

    void
    setToTwo(int *ip)
    {
      *ip = 5;
    }
    ...
    int i;
    setToTwo(&i);
    

  • You can move to the next chunk of memory with ++

    ip++;