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"HTML Tidy for Cygwin (vers 1st April 2002), see www.w3.org">
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"text/html; charset=windows-1252">
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<title>Error and Exception Handling</title>
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<h1>Error and Exception Handling</h1>
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<h2>References</h2>
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<p>The following paper is a good introduction to some of the issues of
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writing robust generic components:</p>
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<blockquote>
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<a href="generic_exception_safety.html">D. Abrahams: ``Exception Safety
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in Generic Components''</a>, originally published in <a href=
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"http://www.springer.de/cgi-bin/search_book.pl?isbn=3-540-41090-2">M.
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Jazayeri, R. Loos, D. Musser (eds.): Generic Programming, Proc. of a
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Dagstuhl Seminar, Lecture Notes on Computer Science. Volume. 1766</a>
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</blockquote>
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<h2>Guidelines</h2>
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<h3>When should I use exceptions?</h3>
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<p>The simple answer is: ``whenever the semantic and performance
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characteristics of exceptions are appropriate.''</p>
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<p>An oft-cited guideline is to ask yourself the question ``is this an
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exceptional (or unexpected) situation?'' This guideline has an attractive
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ring to it, but is usually a mistake. The problem is that one person's
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``exceptional'' is another's ``expected'': when you really look at the
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terms carefully, the distinction evaporates and you're left with no
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guideline. After all, if you check for an error condition, then in some
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sense you expect it to happen, or the check is wasted code.</p>
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<p>A more appropriate question to ask is: ``do we want stack
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unwinding here?'' Because actually handling an exception is likely
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to be significantly slower than executing mainline code, you
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should also ask: ``Can I afford stack unwinding here?'' For
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example, a desktop application performing a long computation might
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periodically check to see whether the user had pressed a cancel
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button. Throwing an exception could allow the operation to be
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cancelled gracefully. On the other hand, it would probably be
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inappropriate to throw and <i>handle</i> exceptions in the inner
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loop of this computation because that could have a significant
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performance impact. The guideline mentioned above has a grain of
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truth in it: in time critical code, throwing an exception
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should <em>be</em> the exception, not the rule.</p>
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<h3>How should I design my exception classes?</h3>
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<ol>
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<li><b>Inherit from <code>std::exception</code></b>. Except in *very*
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rare circumstances where you can't afford the cost of a virtual table,
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<code>std::exception</code> makes a reasonable exception base class,
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and when used universally, allows programmers to catch "everything"
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without resorting to <code>catch(...)</code>. For more about
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<code>catch(...)</code>, see below.</li>
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<li>
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<b><i>Don't</i> embed a std::string object</b> or any other data
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member or base class whose copy constructor could throw an exception.
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That could lead directly to std::terminate() at the throw point.
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Similarly, it's a bad idea to use a base or member whose ordinary
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constructor(s) might throw, because, though not necessarily fatal to
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your program, you may report a different exception than intended from
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a <i>throw-expression</i> that includes construction such as:
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<blockquote>
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<pre>
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throw some_exception();
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</pre>
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</blockquote>
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<p>There are various ways to avoid copying string objects when
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exceptions are copied, including embedding a fixed-length buffer in
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the exception object, or managing strings via reference-counting.
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However, consider the next point before pursuing either of these
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approaches.</p>
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</li>
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<li><b>Format the <code>what()</code> message on demand</b>, if you
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feel you really must format the message. Formatting an exception error
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message is typically a memory-intensive operation that could
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potentially throw an exception. This is an operation best delayed until
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after stack unwinding has occurred, and presumably, released some
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resources. It's a good idea in this case to protect your
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<code>what()</code> function with a <code>catch(...)</code> block so
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that you have a fallback in case the formatting code throws</li>
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<li><b>Don't worry <i>too</i> much about the <code>what()</code>
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message</b>. It's nice to have a message that a programmer stands a
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chance of figuring out, but you're very unlikely to be able to compose
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a relevant and <i>user</i>-comprehensible error message at the point an
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exception is thrown. Certainly, internationalization is beyond the
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scope of the exception class author. <a href=
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"../people/peter_dimov.htm">Peter Dimov</a> makes an excellent argument
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that the proper use of a <code>what()</code> string is to serve as a
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key into a table of error message formatters. Now if only we could get
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standardized <code>what()</code> strings for exceptions thrown by the
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standard library...</li>
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<li><b>Expose relevant information about the cause of the error</b> in
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your exception class' public interface. A fixation on the
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<code>what()</code> message is likely to mean that you neglect to
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expose information someone might need in order to make a coherent
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message for users. For example, if your exception reports a numeric
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range error, it's important to have the actual numbers involved
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available <i>as numbers</i> in the exception class' public interface
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where error reporting code can do something intelligent with them. If
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you only expose a textual representation of those numbers in the
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<code>what()</code> string, you will make life very difficult for
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programmers who need to do something more (e.g. subtraction) with them
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than dumb output.</li>
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<li><b>Make your exception class immune to double-destruction</b> if
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possible. Unfortunately, several popular compilers occasionally cause
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exception objects to be destroyed twice. If you can arrange for that to
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be harmless (e.g. by zeroing deleted pointers) your code will be more
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robust.</li>
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</ol>
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<h3>What About Programmer Errors?</h3>
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<p>As a developer, if I have violated a precondition of a library I'm
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using, I don't want stack unwinding. What I want is a core dump or the
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equivalent - a way to inspect the state of the program at the exact point
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where the problem was detected. That usually means <tt>assert()</tt> or
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something like it.</p>
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<p>Sometimes it is necessary to have resilient APIs which can stand up to
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nearly any kind of client abuse, but there is usually a significant cost
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to this approach. For example, it usually requires that each object used
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by a client be tracked so that it can be checked for validity. If you
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need that sort of protection, it can usually be provided as a layer on
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top of a simpler API. Beware half-measures, though. An API which promises
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resilience against some, but not all abuse is an invitation to disaster.
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Clients will begin to rely on the protection and their expectations will
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grow to cover unprotected parts of the interface.</p>
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<p><b>Note for Windows developers</b>: unfortunately, the native
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exception-handling used by most Windows compilers actually throws an
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exception when you use <tt>assert()</tt>. Actually, this is true of other
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programmer errors such as segmentation faults and divide-by-zero errors.
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One problem with this is that if you use JIT (Just In Time) debugging,
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there will be collateral exception-unwinding before the debugger comes up
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because <code>catch(...)</code> will catch these not-really-C++
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exceptions. Fortunately, there is a simple but little-known workaround,
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which is to use the following incantation:</p>
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<blockquote>
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<pre>
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extern "C" void straight_to_debugger(unsigned int, EXCEPTION_POINTERS*)
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{
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throw;
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}
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extern "C" void (*old_translator)(unsigned, EXCEPTION_POINTERS*)
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= _set_se_translator(straight_to_debugger);
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</pre>
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</blockquote>
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This technique doesn't work if the SEH is raised from within a catch
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block (or a function called from within a catch block), but it still
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eliminates the vast majority of JIT-masking problems.
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<h3>How should I handle exceptions?</h3>
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<p>Often the best way to deal with exceptions is to not handle them at
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all. If you can let them pass through your code and allow destructors to
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handle cleanup, your code will be cleaner.</p>
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<h4>Avoid <code>catch(...)</code> when possible</h4>
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Unfortunately, operating systems other than Windows also wind non-C++
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"exceptions" (such as thread cancellation) into the C++ EH machinery, and
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there is sometimes no workaround corresponding to the
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<code>_set_se_translator</code> hack described above. The result is that
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<code>catch(...)</code> can have the effect of making some unexpected
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system notification at a point where recovery is impossible look just
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like a C++ exception thrown from a reasonable place, invalidating the
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usual safe assumptions that destructors and catch blocks have taken valid
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steps to ensure program invariants during unwinding.
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<p>I reluctantly concede this point to Hillel Y. Sims, after many
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long debates in the newsgroups: until all OSes are "fixed", if
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every exception were derived from <code>std::exception</code> and
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everyone substituted
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<code>catch(std::exception&)</code> for <code>catch(...)</code>, the
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world would be a better place.</p>
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<p>Sometimes, <code>catch(...)</code>, is still the most appropriate
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pattern, in spite of bad interactions with OS/platform design choices. If
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you have no idea what kind of exception might be thrown and you really
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<i>must</i> stop unwinding it's probably still your best bet. One obvious
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place where this occurs is at language boundaries.</p>
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<hr>
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<p>© Copyright David Abrahams 2001-2003. All rights reserved.</p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->
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6 August, 2003<!--webbot bot="Timestamp" endspan i-checksum="34359" -->
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</p>
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</body>
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</html>
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