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F.3. The #define Preprocessor Directive: Symbolic Constants

F 3 The #define Preprocessor Directive Symbolic Constants

The #define preprocessor directive creates symbolic constantsconstants represented as symbolsand macrosoperations defined as symbols. The #define preprocessor directive format is


#define identifier replacement-text

When this line appears in a file, all subsequent occurrences (except those inside a string) of identifier in that file will be replaced by replacement-text before the program is compiled. For example,

#define PI 3.14159

replaces all subsequent occurrences of the symbolic constant PI with the numeric constant 3.14159. Symbolic constants enable the programmer to create a name for a constant and use the name throughout the program. Later, if the constant needs to be modified throughout the program, it can be modified once in the #define preprocessor directiveand when the program is recompiled, all occurrences of the constant in the program will be modified. [Note: Everything to the right of the symbolic constant name replaces the symbolic constant. For example, #define PI = 3.14159 causes the preprocessor to replace every occurrence of PI with = 3.14159. Such replacement is the cause of many subtle logic and syntax errors.] Redefining a symbolic constant with a new value without first undefining it is also an error. Note that const variables in C++ are preferred over symbolic constants. Constant variables have a specific data type and are visible by name to a debugger. Once a symbolic constant is replaced with its replacement text, only the replacement text is visible to a debugger. A disadvantage of const variables is that they might require a memory location of their data type sizesymbolic constants do not require any additional memory.

Common Programming Error F.2

Using symbolic constants in a file other than the file in which the symbolic constants are defined is a compilation error (unless they are #included from a header file).

Good Programming Practice F.1

Using meaningful names for symbolic constants helps make programs more self-documenting.


Introduction to Computers, the Internet and World Wide Web

Introduction to C++ Programming

Introduction to Classes and Objects

Control Statements: Part 1

Control Statements: Part 2

Functions and an Introduction to Recursion

Arrays and Vectors

Pointers and Pointer-Based Strings

Classes: A Deeper Look, Part 1

Classes: A Deeper Look, Part 2

Operator Overloading; String and Array Objects

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

Templates

Stream Input/Output

Exception Handling

File Processing

Class string and String Stream Processing

Web Programming

Searching and Sorting

Data Structures

Bits, Characters, C-Strings and structs

Standard Template Library (STL)

Other Topics

Appendix A. Operator Precedence and Associativity Chart

Appendix B. ASCII Character Set

Appendix C. Fundamental Types

Appendix D. Number Systems

Appendix E. C Legacy Code Topics

Appendix F. Preprocessor

Appendix G. ATM Case Study Code

Appendix H. UML 2: Additional Diagram Types

Appendix I. C++ Internet and Web Resources

Appendix J. Introduction to XHTML

Appendix K. XHTML Special Characters

Appendix L. Using the Visual Studio .NET Debugger

Appendix M. Using the GNU C++ Debugger

Bibliography

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C++ How to Program
C++ How to Program (5th Edition)
ISBN: 0131857576
EAN: 2147483647
Year: 2004
Pages: 627
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