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(cpp-295.info)Deleted Code

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Keeping Deleted Code for Future Reference
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   If you replace or delete a part of the program but want to keep the
old code around as a comment for future reference, the easy way to do
this is to put `#if 0' before it and `#endif' after it.  This is better
than using comment delimiters `/*' and `*/' since those won't work if
the code already contains comments (C comments do not nest).

   This works even if the code being turned off contains conditionals,
but they must be entire conditionals (balanced `#if' and `#endif').

   Conversely, do not use `#if 0' for comments which are not C code.
Use the comment delimiters `/*' and `*/' instead.  The interior of `#if
0' must consist of complete tokens; in particular, singlequote
characters must balance.  But comments often contain unbalanced
singlequote characters (known in English as apostrophes).  These
confuse `#if 0'.  They do not confuse `/*'.