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Thinking in C++
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Introduction to pointers

Whenever you run a program, it is first loaded (typically from disk) into the computer’s memory. Thus, all elements of your program are located somewhere in memory. Memory is typically laid out as a sequential series of memory locations; we usually refer to these locations as eight-bit bytes but actually the size of each space depends on the architecture of the particular machine and is usually called that machine’s word size. Each space can be uniquely distinguished from all other spaces by its address. For the purposes of this discussion, we’ll just say that all machines use bytes that have sequential addresses starting at zero and going up to however much memory you have in your computer.

Since your program lives in memory while it’s being run, every element of your program has an address. Suppose we start with a simple program:

//: C03:YourPets1.cpp
#include <iostream>
using namespace std;

int dog, cat, bird, fish;

void f(int pet) {
  cout << "pet id number: " << pet << endl;

int main() {
  int i, j, k;
} ///:~

Each of the elements in this program has a location in storage when the program is running. Even the function occupies storage. As you’ll see, it turns out that what an element is and the way you define it usually determines the area of memory where that element is placed.

There is an operator in C and C++ that will tell you the address of an element. This is the &’ operator. All you do is precede the identifier name with ‘&’ and it will produce the address of that identifier. YourPets1.cpp can be modified to print out the addresses of all its elements, like this:

//: C03:YourPets2.cpp
#include <iostream>
using namespace std;

int dog, cat, bird, fish;

void f(int pet) {
  cout << "pet id number: " << pet << endl;

int main() {
  int i, j, k;
  cout << "f(): " << (long)&f << endl;
  cout << "dog: " << (long)&dog << endl;
  cout << "cat: " << (long)&cat << endl;
  cout << "bird: " << (long)&bird << endl;
  cout << "fish: " << (long)&fish << endl;
  cout << "i: " << (long)&i << endl;
  cout << "j: " << (long)&j << endl;
  cout << "k: " << (long)&k << endl;
} ///:~

The (long) is a cast. It says “Don’t treat this as if it’s normal type, instead treat it as a long.” The cast isn’t essential, but if it wasn’t there, the addresses would have been printed out in hexadecimal instead, so casting to a long makes things a little more readable.

The results of this program will vary depending on your computer, OS, and all sorts of other factors, but it will always give you some interesting insights. For a single run on my computer, the results looked like this:

f(): 4198736
dog: 4323632
cat: 4323636
bird: 4323640
fish: 4323644
i: 6684160
j: 6684156
k: 6684152

You can see how the variables that are defined inside main( ) are in a different area than the variables defined outside of main( ); you’ll understand why as you learn more about the language. Also, f( ) appears to be in its own area; code is typically separated from data in memory.

Another interesting thing to note is that variables defined one right after the other appear to be placed contiguously in memory. They are separated by the number of bytes that are required by their data type. Here, the only data type used is int, and cat is four bytes away from dog, bird is four bytes away from cat, etc. So it would appear that, on this machine, an int is four bytes long.

Other than this interesting experiment showing how memory is mapped out, what can you do with an address? The most important thing you can do is store it inside another variable for later use. C and C++ have a special type of variable that holds an address. This variable is called a pointer.

The operator that defines a pointer is the same as the one used for multiplication: ‘*’. The compiler knows that it isn’t multiplication because of the context in which it is used, as you will see.

When you define a pointer, you must specify the type of variable it points to. You start out by giving the type name, then instead of immediately giving an identifier for the variable, you say “Wait, it’s a pointer” by inserting a star between the type and the identifier. So a pointer to an int looks like this:

int* ip; // ip points to an int variable

The association of the ‘*’ with the type looks sensible and reads easily, but it can actually be a bit deceiving. Your inclination might be to say “intpointer” as if it is a single discrete type. However, with an int or other basic data type, it’s possible to say:

int a, b, c;

whereas with a pointer, you’d like to say:

int* ipa, ipb, ipc;

C syntax (and by inheritance, C++ syntax) does not allow such sensible expressions. In the definitions above, only ipa is a pointer, but ipb and ipc are ordinary ints (you can say that “* binds more tightly to the identifier”). Consequently, the best results can be achieved by using only one definition per line; you still get the sensible syntax without the confusion:

int* ipa;
int* ipb;
int* ipc;

Since a general guideline for C++ programming is that you should always initialize a variable at the point of definition, this form actually works better. For example, the variables above are not initialized to any particular value; they hold garbage. It’s much better to say something like:

int a = 47;
int* ipa = &a;

Now both a and ipa have been initialized, and ipa holds the address of a.

Once you have an initialized pointer, the most basic thing you can do with it is to use it to modify the value it points to. To access a variable through a pointer, you dereference the pointer using the same operator that you used to define it, like this:

*ipa = 100;

Now a contains the value 100 instead of 47.

These are the basics of pointers: you can hold an address, and you can use that address to modify the original variable. But the question still remains: why do you want to modify one variable using another variable as a proxy?

For this introductory view of pointers, we can put the answer into two broad categories:

  1. To change “outside objects” from within a function. This is perhaps the most basic use of pointers, and it will be examined here.
  2. To achieve many other clever programming techniques, which you’ll learn about in portions of the rest of the
    Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire