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Pointers And Memory - Contents

Basic Pointers


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    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.

   A pointer works a little differently— it does not store a simple value directly. Instead, a pointer stores a reference to another value. The variable the pointer refers to is sometimes known as its "pointee". In a drawing, a pointer is a box which contains the beginning of an arrow which leads to its pointee. (There is no single, official, word for the concept of a pointee — pointee is just the word used in these explanations.)

   The following drawing shows two variables: num and numPtr. The simple variable num contains the value 42 in the usual way. The variable numPtr is a pointer which contains a reference to the variable num. The numPtr variable is the pointer and num is its pointee. What is stored inside of numPtr? Its value is not an int. Its value is a reference to an int.

   Pointer Variables Pointer variables are declared just like any other variable. The declaration gives the type and name of the new variable and reserves memory to hold its value. The declaration does not assign a pointee for the pointer — the pointer starts out with a bad value.
int* numPtr; // Declare the int* (pointer to int) variable "numPtr".
// This allocates space for the pointer, but not the pointee.
// The pointer starts out "bad".

   Pointer Type Syntax A pointer type in C is just the pointee type followed by a asterisk (*)...

int* type: pointer to int
float* type: pointer to float
struct fraction* type: pointer to struct fraction
struct fraction** type: pointer to struct fraction*

   Pointer Dereference The "dereference" operation follows a pointer's reference to get the value of its pointee. The value of the dereference of numPtr above is 42. When the dereference operation is used correctly, it's simple. It just accesses the value of the pointee. The only restriction is that the pointer must have a pointee for the dereference to access. Almost all bugs in pointer code involve violating that one restriction. A pointer must be assigned a pointee before dereference operations will work.

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