Definitions
The preprocessor is a part of the C++ compiler that deals with the source code of a program before the rest of the compiler ever sees that source code; thus, the name "preprocessor".
A preprocessor directive is a command telling the preprocessor to handle the following source code in a special manner.
A preprocessor symbol is a constant value similar to a const but is known only to the preprocessor, not to the rest of the compiler. The rules for naming preprocessor symbols are the same as those for other identifiers, but it is customary to use all uppercase letters in preprocessor symbols so that they can be readily distinguished from other identifiers.
The #ifndef preprocessor directive tells the preprocessor to check whether a particular ...
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