Deleted Code - Red Hat ENTERPRISE LINUX 4 - USING CPP Using Instructions

Chapter 4. Conditionals 41
stands for "else if". Like , it goes in the middle of a conditional group and subdivides
#elif #else
it; it does not require a matching of its own. Like , the directive includes an
#endif #if #elif
expression to be tested. The text following the is processed only if the original -condition
#elif #if
failed and the condition succeeds.
#elif
More than one can go in the same conditional group. Then the text after each is pro-
#elif #elif
cessed only if the condition succeeds after the original and all previous directives
#elif #if #elif
within it have failed.
is allowed after any number of directives, but may not follow .
#else #elif #elif #else

4.3. Deleted Code

If you replace or delete a part of the program but want to keep the old code around for future reference,
you often cannot simply comment it out. Block comments do not nest, so the first comment inside the
old code will end the commenting-out. The probable result is a flood of syntax errors.
One way to avoid this problem is to use an always-false conditional instead. For instance, put
#if 0
before the deleted code and after it. This works even if the code being turned off contains
#endif
conditionals, but they must be entire conditionals (balanced and ).
#if #endif
Some people use instead. This is risky, because might be accidentally de-
#ifdef notdef notdef
fined as a macro, and then the conditional would succeed. can be counted on to fail.
#if 0
Do not use for comments which are not C code. Use a real comment, instead. The interior
#if 0
of must consist of complete tokens; in particular, single-quote characters must balance. Com-
#if 0
ments often contain unbalanced single-quote characters (known in English as apostrophes). These
confuse . They don't confuse .
#if 0 /*