can't happen


    The traditional program comment for code executed under a condition that
    should never be true, for example a file size computed as negative. Often,
    such a condition being true indicates data corruption or a faulty
    algorithm; it is almost always handled by emitting a fatal error message
    and terminating or crashing, since there is little else that can be done.
    Some case variant of ?can't happen? is also often the text emitted if the
    ?impossible? error actually happens! Although ?can't happen? events are
    genuinely infrequent in production code, programmers wise enough to check
    for them habitually are often surprised at how frequently they are
    triggered during development and how many headaches checking for them turns
    out to head off. See also firewall code (sense 2).

can't happen
This can't happen

    The traditional program comment for code
   executed under a condition that should never be true, for
   example a file size computed as negative.  Often, such a
   condition being true indicates data corruption or a faulty
   algorithm; it is almost always handled by emitting a fatal
   error message and terminating or crashing, since there is
   little else that can be done.

   Some case variant of "can't happen" is also often the text
   emitted if the "impossible" error actually happens.  Although
   "can't happen" events are genuinely infrequent in production
   code, programmers wise enough to check for them habitually are
   often surprised at how frequently they are triggered during
   development and how many headaches checking for them turns out
   to head off.

   See also firewall code, professional programming.

   [Jargon File]

   (1996-05-10)