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In Chapter 8, you learned that the C compiler allows you to
define macro symbols using #define.
The C compiler's preprocessor also allows you to specify that certain
lines of code are to be compiled only if a specified macro symbol has been
defined. Suppose, for example, that you want certain optional lines of
code to be compiled only if you have previously defined VERBOSE to
be a macro symbol; otherwise, you want your C compiler to ignore those
optional lines entirely.
To arrange for such conditional compilation, you simply sandwich
those optional lines of code between a line reading
#ifdef VERBOSE and a line reading #endif:
#ifdef VERBOSE ... | ... | <-- Compile if VERBOSE has been defined ... | #endif
Symmetrically, the lines between the compiler instructions
#ifndef and #endif are ignored if VERBOSE is
defined; otherwise, they are compiled:
#ifndef VERBOSE ... | ... | <-- Compile if VERBOSE has NOT been defined ... | #endif