# Error and Exception Handling

## References

The following paper is a good introduction to some of the issues of writing robust generic components:

> D. Abrahams: ``Exception Safety in Generic Components'', originally published in M. Jazayeri, R. Loos, D. Musser (eds.): Generic Programming, Proc. of a Dagstuhl Seminar, Lecture Notes on Computer Science. Volume. 1766

## Guidelines

### When should I use exceptions?

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.

### How should I design my exception classes?

1. Derive your exception class from `std::exception`. Except in *very* rare circumstances where you can't afford the cost of a virtual table, `std::exception` makes a reasonable exception base class, and when used universally, allows programmers to catch "everything" without resorting to `catch(...)`. For more about `catch(...)`, see below.
2. Use virtual inheritance. This insight is due to Andrew Koenig. Using virtual inheritance from your exception's base class(es) prevents ambiguity problems at the catch-site in case someone throws an exception derived from multiple bases which have a base class in common:

       #include <iostream>
       struct my_exc1 : std::exception { char const* what() const throw(); };
       struct my_exc2 : std::exception { char const* what() const throw(); };
       struct your_exc3 : my_exc1, my_exc2 {};

       int main()
       {
          try { throw your_exc3(); }
          catch(std::exception const& e) {}
          catch(...) { std::cout << "whoops!" << std::endl; }
       }

   The program above prints `"whoops"` because the C++ runtime can't resolve which `exception` instance to match in the first catch clause.
3. Don't embed a std::string object or any other data member or base class whose copy constructor could throw an exception. That could lead directly to std::terminate() at the throw point. Similarly, it's a bad idea to use a base or member whose ordinary constructor(s) might throw, because, though not necessarily fatal to your program, you may report a different exception than intended from a throw-expression that includes construction such as:

   >     throw some_exception();

   There are various ways to avoid copying string objects when exceptions are copied, including embedding a fixed-length buffer in the exception object, or managing strings via reference-counting. However, consider the next point before pursuing either of these approaches.

4. Format the `what()` message on demand, if you feel you really must format the message. Formatting an exception error message is typically a memory-intensive operation that could potentially throw an exception. This is an operation best delayed until after stack unwinding has occurred, and presumably, released some resources. It's a good idea in this case to protect your `what()` function with a `catch(...)` block so that you have a fallback in case the formatting code throws
5. Don't worry too much about the `what()` message. It's nice to have a message that a programmer stands a chance of figuring out, but you're very unlikely to be able to compose a relevant and user-comprehensible error message at the point an exception is thrown. Certainly, internationalization is beyond the scope of the exception class author. Peter Dimov makes an excellent argument that the proper use of a `what()` string is to serve as a key into a table of error message formatters. Now if only we could get standardized `what()` strings for exceptions thrown by the standard library...
6. Expose relevant information about the cause of the error in your exception class' public interface. A fixation on the `what()` message is likely to mean that you neglect to expose information someone might need in order to make a coherent message for users. For example, if your exception reports a numeric range error, it's important to have the actual numbers involved available as numbers in the exception class' public interface where error reporting code can do something intelligent with them. If you only expose a textual representation of those numbers in the `what()` string, you will make life very difficult for programmers who need to do something more (e.g. subtraction) with them than dumb output.
7. Make your exception class immune to double-destruction if possible. Unfortunately, several popular compilers occasionally cause exception objects to be destroyed twice. If you can arrange for that to be harmless (e.g. by zeroing deleted pointers) your code will be more robust.

### What About Programmer Errors?

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 exception-unwinding 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.

### How should I handle exceptions?

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.

#### Avoid `catch(...)` when possible

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.

© Copyright David Abrahams 2001-2003. All rights reserved.

Revised 21 August, 2003