Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++
Prev Contents / Index Next

The preprocessor directives #define, #ifdef, and #endif

The preprocessor directive #define can be used to create compile-time flags. You have two choices: you can simply tell the preprocessor that the flag is defined, without specifying a value:

#define FLAG

or you can give it a value (which is the typical C way to define a constant):

#define PI 3.14159

In either case, the label can now be tested by the preprocessor to see if it has been defined:

#ifdef FLAG

This will yield a true result, and the code following the #ifdef will be included in the package sent to the compiler. This inclusion stops when the preprocessor encounters the statement



#endif // FLAG

Any non-comment after the #endif on the same line is illegal, even though some compilers may accept it. The #ifdef/#endif pairs may be nested within each other.

The complement of #define is #undef (short for “un-define”), which will make an #ifdef statement using the same variable yield a false result. #undef will also cause the preprocessor to stop using a macro. The complement of #ifdef is #ifndef, which will yield a true if the label has not been defined (this is the one we will use in header files).

There are other useful features in the C preprocessor. You should check your local documentation for the full set.

Thinking in C++
Prev Contents / Index Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire