Title: Error and Exception Handling

The simple answer is: "whenever the semantic and performance characteristics of exceptions are appropriate."

An oft-cited guideline is to ask yourself the question "is this an exceptional (or unexpected) situation?" This guideline has an attractive ring to it, but is usually a mistake. The problem is that one person's "exceptional" is another's "expected": when you really look at the terms carefully, the distinction evaporates and you're left with no guideline. After all, if you check for an error condition, then in some sense you expect it to happen, or the check is wasted code.

A more appropriate question to ask is: "do we want stack unwinding here?" Because actually handling an exception is likely to be significantly slower than executing mainline code, you should also ask: "Can I afford stack unwinding here?" For example, a desktop application performing a long computation might periodically check to see whether the user had pressed a cancel button. Throwing an exception could allow the operation to be cancelled gracefully. On the other hand, it would probably be inappropriate to throw and handle exceptions in the inner loop of this computation because that could have a significant performance impact. The guideline mentioned above has a grain of truth in it: in time critical code, throwing an exception should be the exception, not the rule.

As a developer, if I have violated a precondition of a library I'm using, I don't want stack unwinding. What I want is a core dump or the equivalent - a way to inspect the state of the program at the exact point where the problem was detected. That usually means assert() or something like it.

Sometimes it is necessary to have resilient APIs which can stand up to nearly any kind of client abuse, but there is usually a significant cost to this approach. For example, it usually requires that each object used by a client be tracked so that it can be checked for validity. If you need that sort of protection, it can usually be provided as a layer on top of a simpler API. Beware half-measures, though. An API which promises resilience against some, but not all abuse is an invitation to disaster. Clients will begin to rely on the protection and their expectations will grow to cover unprotected parts of the interface.

Note for Windows developers: unfortunately, the native exception-handling used by most Windows compilers actually throws an exception when you use assert(). Actually, this is true of other programmer errors such as segmentation faults and divide-by-zero errors. One problem with this is that if you use JIT (Just In Time) debugging, there will be collateral exceptionunwinding before the debugger comes up because catch(...) will catch these not-really-C++ exceptions. Fortunately, there is a simple but little-known workaround, which is to use the following incantation:

extern "C" void straight_to_debugger(
               unsigned int,
               EXCEPTION_POINTERS*) {
  throw;
}

extern "C" void
  (*old_translator)(unsigned,
                    EXCEPTION_POINTERS*)
  = _set_se_translator(
                    straight_to_debugger);

This technique doesn't work if the SEH is raised from within a catch block (or a function called from within a catch block), but it still eliminates the vast majority of JIT-masking problems.

Often the best way to deal with exceptions is to not handle them at all. If you can let them pass through your code and allow destructors to handle cleanup, your code will be cleaner.

Unfortunately, operating systems other than Windows also wind non-C++ "exceptions" (such as thread cancellation) into the C++ EH machinery, and there is sometimes no workaround corresponding to the _set_se_translator hack described above. The result is that catch(...) can have the effect of making some unexpected system notification at a point where recovery is impossible look just like a C++ exception thrown from a reasonable place, invalidating the usual safe assumptions that destructors and catch blocks have taken valid steps to ensure program invariants during unwinding.

I reluctantly concede this point to Hillel Y. Sims, after many long debates in the newsgroups: until all OSes are "fixed", if every exception were derived from std::exception and everyone substituted catch(std::exception&) for catch(...), the world would be a better place.

Sometimes, catch(...), is still the most appropriate pattern, in spite of bad interactions with OS/platform design choices. If you have no idea what kind of exception might be thrown and you really must stop unwinding it's probably still your best bet. One obvious place where this occurs is at language boundaries.