== The problem with _#ifdef_

Something that crystallized in the process of writing [[the earlier
Bourne shell quoting entry BourneQuotingII]] is that _#ifdef_ abuse
leads to bad results for the same fundamental reason that multiple
levels of escaping are bad: after a certain point, people can no longer
clearly see what the real code will look like.

When you can't see what the code looks like, it's really hard to make
sensible changes; either you make blind stabs or you have to carefully
reconstruct the actual code, usually by hand. Either process is error
prone, and it's easy to fool yourself, and you are effectively doing
remote control programming (with mushy feedback).

Ironically I suspect that the really dangerous _#ifdef_'d code is not
the code that is completely snarled up, but the code that is halfway
there. Code that is a complete mess is clearly beyond understanding,
but code that is only half-overgrown with _#ifdef_s tempts you into
thinking that you can follow it when you actually can't.

(The worst _#ifdef_'d code that I've personally encountered and
haven't frantically scrubbed out of my brain is the _xterm_ code,
which is a shining example of what not to do to make a portable
program.)