The C Preprocessor
A macro is a fragment of code which has been given a name.
Whenever the name is used, it is replaced by the contents of the macro.
There are two kinds of macros. They differ mostly in what they look
like when they are used. Object-like macros resemble data objects
when used, function-like macros resemble function calls.
You may define any valid identifier as a macro, even if it is a C
keyword. The preprocessor does not know anything about keywords. This
can be useful if you wish to hide a keyword such as
const from an
older compiler that does not understand it. However, the preprocessor
defined (see section 4.2.3 Defined) can never be defined as a
macro, and C++'s named operators (see section 3.7.4 C++ Named Operators) cannot be
macros when you are compiling C++.