Juding whether an exception is exceptional

It's a pretty popular and well known phrase that you should "only catch/throw exceptions which are exceptional". However, how is an "exceptional" exception determined?

For example, a bad password is very routine in logging into a service, so this is not exceptional. Statistics for a web app would probably show something like one bad开发者_开发知识库 login attempt for every 5 attempts (from no specific user). Likewise, with attempting to go to a checkout with a basket in an online store, this could be very commmon (especially for new users). However, a file not found could go either way. I usually work along the lines that if a method is missing something to do its work, throw an exception, but then it gets a little confusing here. In some cases, a file not found could be common (e.g. a file share used by many users with no tight controls), compared to a very locked down production environment missing a file, which would be exceptional.

Is this the right way to deduce between whether an exception is exceptional or not? I can easily filter things like no network connection etc as exceptional, but some cases are hard to judge. Is it subjective?

Thanks

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I think it's pretty subjective, honestly, so I prefer to avoid that method of figuring out when I should use exceptions.

Instead, I prefer to consider three things:

1. Is it likely that I might want to let the call stack unwind more than one level?
2. Is there another way? (Return null or an error code, etc.) If so, do I have even the slightest performance concern?
3. If neither of those lead to a clear decision, which is easier to read by someone who has to maintain the code?

If #1 is true, and I don't have a MAJOR performance concern, I will probably opt to use exceptions because it will speed up my development time not to have to code return codes (and manually code the logic to have them propagate up the call stack if needed). When you use exceptions, call stack unwinding is free of charge for development time.

If #2 is true, and either I'm not going more than one frame (maybe two?) up the call stack or I have a serious performance concern (in a tight loop, for example), then I'll try really hard to find another way that doesn't involve exceptions.

Exceptions are only a tool for programmers in a language which supports them. I don't believe they have to have any intrinsic value as to what is "exceptional" or not. Instead, I say use them when they are the best tool for the job.