[Interest] More on Exceptions

[Turtle Creek Software]

Thiago, I'll describe what happens when our typical exception is thrown,
since conditions are very different from what you describe.

Before any method uses a pointer, it first does a sanity check. If nullptr
it shows a dialog with file/line number and what failed, then throws an
exception to avoid crashing.  There are 7000+ checks, plus another 1000+
assertions for other bad states. 99% will never be called. Some throws are
caught internally but most go out to the event loop, which has a try/catch
block.

The app has a couple megabytes of just file/line number text, since it
needs to be compiled in. The original plan was to remove them in release
build, but they help for user bug reports so they're permanent. Users tell
us exactly where to find the problem.

A few exceptions are fatal and just terminate.  The database is never in
unstable state so we can leave safely at any time. Actually it's unstable
briefly during commits but nothing throws in there so the only danger is
power failures.

There probably are apps that need cleanup on terminate, but ours isn't one.

In practice, 99% of throws are from something trivial/stupid- a missing
field, or a link to something that doesn't need to be linked.  They are
caught in the event loop and users can safely continue working. In rare
cases they may see a ton of weird messages and need to force-quit.

As the stack unwinds after a throw, in theory it should tidy everything via
scope endings and destructors.  In practice, maybe there are leaks or other
bugs along the way. But in theory, those bugs would show up anyhow when the
stack unwinds from normal use.  In practice, the setup has worked well for
20+ years. Users see helpful error messages rather than crashes.  Maybe the
exception throws cause subtle, unknown problems but no worse than the usual
problems in any app.

I think Qt should enable exceptions in QWidget etc by default, strengthen
the disclaimer a bit, and feel no obligation to survive throws in perfect
condition. Pass them along and let developers deal with the consequences.
Exceptions should be for weird problems, and if things get weirder, so
what? Based on our experience in x86 it probably will be just fine.

Thanks,
Dennis Kolva
Programming Director
TurtleSoft.com
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