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+<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Exceptions</title><meta name="generator" content="DocBook XSL Stylesheets V1.7
5.2" /><meta name="keywords" content=" C++ , exception , error , exception neutrality , exception safety , exception propagation , -fno-exceptions " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_concurrency.html" title="Concurrency" /><link rel="next" href="debug.html" title="Debugging Support" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Exceptions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Exceptions"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.exceptions"></a>Exceptions</h2></div></div></div><p>
The C++ language provides language support for stack unwinding
with <code class="literal">try</code> and <code class="literal">catch</code> blocks and
the <code class="literal">throw</code> keyword.
These are very powerful constructs, and require some thought when
applied to the standard library in order to yield components that work
efficiently while cleaning up resources when unexpectedly killed via
-exceptional circumstances.
+exceptional circumstances.
</p><p>
Two general topics of discussion follow:
exception neutrality and exception safety.
-</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.safety"></a>Exception Safety</h3></div></div></div><p>
- What is exception-safe code?
+</p><div class="sect2" title="Exception Safety"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.safety"></a>Exception Safety</h3></div></div></div><p>
+ What is exception-safe code?
</p><p>
Will define this as reasonable and well-defined behavior by classes
and functions from the standard library when used by user-defined
Using the layered approach from Abrahams, can classify library
components as providing set levels of safety. These will be called
exception guarantees, and can be divided into three categories.
- </p><div class="itemizedlist"><ul
type="disc"><li><p>
+ </p><div class="itemizedlist"><ul
class="itemizedlist" type="disc"><li class="listitem"><p>
One. Don't throw.
</p><p>
As specified in 23.2.1 general container requirements. Applicable
Member
functions <code class="function">erase</code>, <code class="function">pop_back</code>, <code class="function">pop_front</code>, <code class="function">swap</code>, <code class="function">clear</code>. And <span class="type">iterator</span>
copy constructor and assignment operator.
- </p></li><li><p>
+ </p></li><li
class="listitem"><p>
Two. Don't leak resources when exceptions are thrown. This is
- also referred to as the “<span class="quote">basic</span>” exception safety guarantee.
+ also referred to as the <span class="quote">“<span class="quote">basic</span>”</span> exception safety guarantee.
</p><p>
This applicable throughout the standard library.
- </p></li><li><p>
+ </p></li><li
class="listitem"><p>
Three. Commit-or-rollback semantics. This is
- referred to as “<span class="quote">strong</span>” exception safety guarantee.
+ referred to as <span class="quote">“<span class="quote">strong</span>”</span> exception safety guarantee.
</p><p>
As specified in 23.2.1 general container requirements. Applicable
to container and string classes.
Member functions <code class="function">insert</code> of a single
element, <code class="function">push_back</code>, <code class="function">push_front</code>,
and <code class="function">rehash</code>.
- </p></li></ul></div></div><div class="sect2"
lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.propagating"></a>Exception Neutrality</h3></div></div></div><p>
+ </p></li></ul></div></div><div class="sect2"
title="Exception Neutrality"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.propagating"></a>Exception Neutrality</h3></div></div></div><p>
Simply put, once thrown an exception object should continue in
flight unless handled explicitly. In practice, this means
propagating exceptions should not be swallowed in
Unfortunately, this tends to be more of a guideline than a strict
rule as applied to the standard library. As such, the following is
a list of known problem areas where exceptions are not propagated.
- </p><div class="itemizedlist"><ul
type="disc"><li><p>
+ </p><div class="itemizedlist"><ul
class="itemizedlist" type="disc"><li class="listitem"><p>
Input/Output
</p><p>
The destructor <code class="function">ios_base::Init::~Init()</code>
When closing the underlying
file, <code class="function">basic_filebuf::close</code> will swallow
(non-cancellation) exceptions thrown and return <code class="literal">NULL</code>.
- </p></li><li><p>
+ </p></li><li
class="listitem"><p>
Thread
</p><p>
The constructors of <code class="classname">thread</code> that take a
callable function argument swallow all exceptions resulting from
executing the function argument.
- </p></li></ul></div></div><div class="sect2"
lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"></a>Doing without</h3></div></div></div><p>
+ </p></li></ul></div></div><div class="sect2"
title="Doing without"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"></a>Doing without</h3></div></div></div><p>
C++ is a language that strives to be as efficient as is possible
in delivering features. As such, considerable care is used by both
language implementer and designers to make sure unused features
use <code class="literal">-fno-exceptions</code>. If you have some code that
uses <code class="literal">try</code> or <code class="literal">catch</code>, you
shouldn't use <code class="literal">-fno-exceptions</code>.
- </p><p>
+ </p><p>
And what it to be gained, tinkering in the back alleys with a
language like this? Exception handling overhead can be measured
in the size of the executable binary, and varies with the
library has been transformed, user code may need modification. User
code that attempts or expects to do error checking on standard
library components compiled with exception handling disabled should
- be evaluated and potentially made conditional.
+ be evaluated and potentially made conditional.
</p><p>
Some issues remain with this approach (see bugzilla entry
25191). Code paths are not equivalent, in
substitution of the C language keyword
<code class="literal">const</code> with the uglified
doppelganger <code class="literal">__const</code>.
-</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.compat"></a>Compatibility</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.c"></a>With <code class="literal">C</code></h4></div></div></div><p>
+</p></div><div class="sect2" title="Compatibility"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.compat"></a>Compatibility</h3></div></div></div><div class="sect3" title="With C"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.c"></a>With <code class="literal">C</code></h4></div></div></div><p>
C language code that is expecting to interoperate with C++ should be
compiled with <code class="literal">-fexceptions</code>. This will make
debugging a C language function called as part of C++-induced stack
getting these details right. For GNU systems, all appropriate parts
of the GNU C library are already compiled
with <code class="literal">-fexceptions</code>.
-</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.posix"></a>With <code class="literal">POSIX</code> thread cancellation</h4></div></div></div><p>
+</p></div><div class="sect3" title="With POSIX thread cancellation"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.posix"></a>With <code class="literal">POSIX</code> thread cancellation</h4></div></div></div><p>
GNU systems re-use some of the exception handling mechanisms to
track control flow for <code class="literal">POSIX</code> thread cancellation.
</p><p>
}
catch(...)
{ this->_M_setstate(ios_base::badbit); }
-</pre></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="using.exceptions.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id618326"></a><p><span class="title"><i>
- System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
- </i>. </span><span class="pagenums">
+</pre></div></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="using.exceptions.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id398527"></a><p><span class="biblioid">
+ <a class="ulink" href="http://www.opengroup.org/austin" target="_top">
+ <em class="citetitle">
+ System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
+ </em>
+ </a>
+ . </span><span class="pagenums">
2.9.5 Thread Cancellation
. </span><span class="copyright">Copyright © 2008
- The Open Group/The Institute of Electrical and Electronics Engineers, Inc.. </span><span class="biblioid">
- <a class="ulink" href="http://www.opengroup.org/austin/" target="_top">
- </a>
- . </span></p></div><div class="biblioentry"><a id="id627763"></a><p><span class="title"><i>
- Error and Exception Handling
- </i>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams </span>. </span><span class="publisher"><span class="publishername">
- Boost
- . </span></span><span class="biblioid">
+ The Open Group/The Institute of Electrical and Electronics
+ Engineers, Inc.
+ . </span></p></div><div class="biblioentry"><a id="id403773"></a><p><span class="biblioid">
<a class="ulink" href="http://www.boost.org/community/error_handling.html" target="_top">
+ <em class="citetitle">
+ Error and Exception Handling
+ </em>
</a>
- . </span></p></div><div class="biblioentry"><a id="id633982"></a><p><span class="title"><i>
- Exception-Safety in Generic Components
- </i>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams</span>. </span><span class="publisher"><span class="publishername">
+ . </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams </span>. </span><span class="publisher"><span class="publishername">
Boost
- . </span></span><span class="biblioid">
+ . </span></span></p></div><div class="biblioentry"><a id="id415460"></a><p><span class="biblioid">
<a class="ulink" href="http://www.boost.org/community/exception_safety.html" target="_top">
+ <em class="citetitle">
+ Exception-Safety in Generic Components
+ </em>
</a>
- . </span></p></div><div class="biblioentry"><a id="id634268"></a><p><span class="title"><i>
- Standard Library Exception Policy
- </i>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
- WG21 N1077
- . </span></span><span class="biblioid">
+ . </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams</span>. </span><span class="publisher"><span class="publishername">
+ Boost
+ . </span></span></p></div><div class="biblioentry"><a id="id441496"></a><p><span class="biblioid">
<a class="ulink" href="www.open-std.org/jtc1/sc22/wg21/docs/papers/1997/N1077.pdf" target="_top">
+ <em class="citetitle">
+ Standard Library Exception Policy
+ </em>
</a>
- . </span></p></div><div class="biblioentry"><a id="id642510"></a><p><span class="title"><i>
- ia64 c++ abi exception handling
- </i>. </span><span class="author"><span class="firstname">Richard</span> <span class="surname">Henderson</span>. </span><span class="publisher"><span class="publishername">
- GNU
- . </span></span><span class="biblioid">
+ . </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
+ WG21 N1077
+ . </span></span></p></div><div class="biblioentry"><a id="id387605"></a><p><span class="biblioid">
<a class="ulink" href="http://gcc.gnu.org/ml/gcc-patches/2001-03/msg00661.html" target="_top">
+ <em class="citetitle">
+ ia64 c++ abi exception handling
+ </em>
</a>
- . </span></p></div><div class="biblioentry"><a id="id636666"></a><p><span class="title"><i>
- Appendix E: Standard-Library Exception Safety
- </i>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="biblioid">
+ . </span><span class="author"><span class="firstname">Richard</span> <span class="surname">Henderson</span>. </span><span class="publisher"><span class="publishername">
+ GNU
+ . </span></span></p></div><div class="biblioentry"><a id="id445074"></a><p><span class="biblioid">
<a class="ulink" href="http://www.research.att.com/~bs/3rd_safe.pdf" target="_top">
+ <em class="citetitle">
+ Appendix E: Standard-Library Exception Safety
+ </em>
</a>
- . </span><
/p></div><div class="biblioentry"><a id="id707620"></a><p><span class="title"><i>
+ . </span><
span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span></p></div><div class="biblioentry" title="Exceptional C++"><a id="id390058"></a><p><span class="title"><i>
Exceptional C++
</i>. </span><span class="pagenums">
Exception-Safety Issues and Techniques
- . </span><span class="author"><span class="firstname">Herb</span> <span class="surname">Sutter</span>. </span></p></div><div class="biblioentry"><a id="id652939"></a><p><span class="title"><i>
- exception_defines.h #defines try/catch
- </i>. </span><span class="subtitle">
- GCC Bug <a class="ulink" href="http://gcc.gnu.org/PR25191" target="_top">25191</a>
- . </span><span class="biblioid">
+ . </span><span class="author"><span class="firstname">Herb</span> <span class="surname">Sutter</span>. </span></p></div><div class="biblioentry"><a id="id445914"></a><p><span class="biblioid">
+ <a class="ulink" href="http://gcc.gnu.org/PR25191" target="_top">
+ <em class="citetitle">
+ GCC Bug 25191: exception_defines.h #defines try/catch
+ </em>
+ </a>
. </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Concurrency </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Debugging Support</td></tr></table></div></body></html>
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