Re: Assertions in principle
On Mar 2, 8:08 am, "jeffc...@yahoo.com" <jeffc...@yahoo.com> wrote:
This might be less of a design issue than a C++ language issue per se,
but I have a problem with assertions. I mean, they work, of course.
But there's something I'm uncomfortable with that's never been
explained to my satisfaction.
I recently read this explanation. "There is an effective litmus test
to differentiate the cases in which you need to use assert and when
you need to use genuine error checking: you use error checking for
things that could happen, even very improbably. You use assert only
for things that you truly believe cannot possibly happen under any
circumstances. An assertion that fails always signals a design or a
programmer error - not a user error."
OK I can buy that. But what keeps going unmentioned is that
assertions are debug-mode only.
The ANSI/ISO C assert, inherited into C++, is disabled by the presence
of the NDEBUG macro. This is not the same thing as being ``debug-mode
only''.
Debug mode is whatever your compiler, build environment and local
conventions dictate debug mode is.
Where I work, both debug and optimized builds have assertions enabled.
C. A. R. Hoare (of quicksort fame) wrote this sometime in the early
1970's:
It is absurd to make elaborate security checks on debugging runs,
when no trust is putin the results, and then remove them in
production runs, when an erroneous result could be expensive
or disastrous. What would we think of a sailing enthusiast who
wears his life-jacket when training on dry land but takes it
off as soon as he goes to sea?
So your argument finds good company, indeed.
I would go so far as to say the original developer's box is precisely
where assertions are NOT needed, because that's the only place where a
debugger is available. (I see how they can come in handy, and force
you to resist making assumptions.) But you really need assertions (or
something) in environments where the debugger isn't available.
You need both. Under debugger, an assertion triggers the type of event
that causes the program to stop so that it can be debugged. Without
the assertion, the program will keep executing; it won't stop until
some fault occurs. By that time, in spite of the debugger being
available at that point, it may be harder to locate the root cause.
Assertions go off closer to the root causes of defects.