The preprocessor is used to modify your program according to the preprocessor directives
in your source code. Preprocessor directives (such as #define) give the preprocessor
specific instructions on how to modify your source code. The preprocessor reads
in all of your include files and the source code you are compiling and creates a
preprocessed version of your source code. This preprocessed version has all of its
macros and constant symbols replaced by their corresponding code and value assignments.
If your source code contains any conditional preprocessor directives (such as #if),
the preprocessor evaluates the condition and modifies your source code accordingly.
The preprocessor contains many features that are powerful to use, such as creating
macros, performing conditional compilation, inserting predefined environment variables
into your code, and turning compiler features on and off. For the professional programmer,
in-depth knowledge of the features of the preprocessor can be one of the keys to
creating fast, efficient programs.