can't happen

programming 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)