ifndef directive
From Cppwiki
#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.
