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93 lines
4.5 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<meta http-equiv="Content-Type" content="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:
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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 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.''
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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.
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<p>A more appropriate question to ask is: ``do we want stack unwinding
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here?'' Because actually handling an exception is likely to be
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significantly slower than executing mainline code, you should also ask:
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``Can I afford stack unwinding here?'' For example, a desktop application
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performing a long computation might periodically check to see whether the
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user had pressed a cancel button. Throwing an exception could allow the
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operation to be cancelled gracefully. On the other hand, it would probably
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be inappropriate to throw and <i>handle</i> exceptions in the inner loop of
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this computation because that would have a significant performance impact.
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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.
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<p>Sometimes it is neccessary 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 to
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this approach. For example, it usually requires that each object used by a
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client be tracked so that it can be checked for validity. If you need that
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sort of protection, it can usually be provided as a layer on top of a
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simpler API. Beware half-measures, though. An API which promises resilience
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against some, but not all abuse is an invitation to disaster. Clients will
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begin to rely on the protection and their expectations will grow to cover
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unprotected parts of the interface.
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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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Fortunately, there is a simple but little-known workaround, which is to use
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the following incantation:
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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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<hr>
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<p>© Copyright David Abrahams 2001. Permission to copy, use, modify,
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sell and distribute this document is granted provided this copyright notice
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appears in all copies. This document is provided "as is" without express or
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implied warranty, and with no claim as to its suitability for any purpose.
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->19 August, 2001<!--webbot bot="Timestamp" endspan i-checksum="34359" -->
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