ifndef directive

#ifndef if a preprocessor directive. It allows for conditional compilaion if a given symbol has not been #defined.

The most common use of #ifndef is in include guards.

Example: #ifndef MYHEADER_H_JAN31_2007_ #define MYHEADER_H_JAN31_2007_

/* Header code goes here */

#endif // #ifndef MYHEADER_H_JAN31_2007_

This example prevents the contents of the #ifndef block from being parsed more than once.

See also: #define, #endif, #if, #ifdef, #include