Valid Pointer Operations

Here is a list of the operations that can properly be performed with pointers.

Creation The initial value of a pointer has three possible sources:

  • A const pointer such as an array name
  • An address obtained by using the address-of operator, &
  • A value obtained by a dynamic memory allocation operator (e.g., new)

Assignment

  • A pointer can be assigned the address stored by a pointer of the same type or of a derived type.
  • A variable of type void* can be assigned a pointer of any type without an explicit cast.
  • A (non-void*) pointer can be assigned the address stored by a pointer of a different (and non-derived) type only with an explicit cast.
  • An array name is a const pointer and cannot be assigned to.
  • A NULL pointer (value 0) can always be assigned to any pointer. (Note: Stroustrup recommends that 0 be used instead of NULL.)

Arithmetic

  • Incrementing and decrementing a pointer: p++ and p--
  • Adding or subtracting an integer: p + k and p - k
  • Such expressions are defined only if the resulting pointer value is within the range of the same array. The only exception to this rule is that a pointer is allowed to point to the memory cell that is one position beyond the end of the array as long as no attempt is made to dereference that address.
  • For subtraction, two pointers that point to two members of an array can be subtracted, yielding an int that represents the number of array elements between the two members.

Comparison

  • Pointers to entries of the same array can be compared with ==, !=, <, >, etc.
  • Any pointer can be compared with 0.

Indirection

  • If p is a pointer of type T*, then *p is a variable of type T and can be used on the left side of an assignment.

Indexing

  • A pointer p can be used with an array index operator p[i] where i is an int.
  • The compiler interprets such an expression as *(p+i).
  • Indexing makes sense and is defined only in the context of an array, but the compiler will not prevent its use with non-array pointers where the results are undefined.

The following bit of code in Example 22.6 demonstrates this last point rather clearly.

Example 22.6. src/arrays/pointerIndex.cpp

#include 
using namespace std;

int main() {
 int x = 23;
 int* px = &x;
 cout << "px[0] = " << px[0] << endl;
 cout << "px[1] = " << px[1] << endl;
 cout << "px[-1] = " << px[-1] << endl;
 return 0;
}
Output:

src/arays> g++ pointerIndex.cc // compile & run on a Sun station
src/arays> a.out
px[0] = 23
px[1] = 5
px[-1] = -268437516

src/arays> g++ pointerIndex.cc // compile & run on a Linux box
src/arays> ./a.out
px[0] = 23
px[1] = -1073743784
px[-1] = -1073743852
src/arays>


Part I: Introduction to C++ and Qt 4

C++ Introduction

Classes

Introduction to Qt

Lists

Functions

Inheritance and Polymorphism

Part II: Higher-Level Programming

Libraries

Introduction to Design Patterns

QObject

Generics and Containers

Qt GUI Widgets

Concurrency

Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments

Bibliography

MP3 Jukebox Assignments



An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268

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