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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 V1.74.0" /><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" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.exception"></a>Exceptions</h2></div></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>Propagating Exceptions aka Exception Neutrality</h3></div></div></div><p>
- </p></div><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>
- </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"></a>Support for <code class="literal">-fno-exceptions</code></h3></div></div></div><p>
- </p></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>
+<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 V1.75.2" /><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.
+</p><p>
+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.
+</p><p>
+Two general topics of discussion follow:
+exception neutrality and exception safety.
+</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
+ classes and functions that are themselves exception safe.
+ </p><p>
+ Please note that using exceptions in combination with templates
+ imposes an additional requirement for exception
+ safety. Instantiating types are required to have destructors that
+ do no throw.
+ </p><p>
+ 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 class="itemizedlist" type="disc"><li class="listitem"><p>
+ One. Don't throw.
+ </p><p>
+ As specified in 23.2.1 general container requirements. Applicable
+ to container and string classes.
+ </p><p>
+ 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 class="listitem"><p>
+ Two. Don't leak resources when exceptions are thrown. This is
+ 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 class="listitem"><p>
+ Three. Commit-or-rollback semantics. This is
+ 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.
+ </p><p>
+ 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" 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
+ gratuitous <code class="literal">catch(...)</code> blocks. Instead,
+ matching <code class="literal">try</code> and <code class="literal">catch</code>
+ blocks should have specific catch handlers and allow un-handed
+ exception objects to propagate. If a
+ terminating <code class="literal">catch(...)</code> blocks exist then it
+ should end with a <code class="literal">throw</code> to re-throw the current
+ exception.
+ </p><p>
+ Why do this?
+ </p><p>
+ By allowing exception objects to propagate, a more flexible
+ approach to error handling is made possible (although not
+ required.) Instead of dealing with an error immediately, one can
+ allow the exception to propagate up until sufficient context is
+ available and the choice of exiting or retrying can be made in an
+ informed manner.
+ </p><p>
+ 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 class="itemizedlist" type="disc"><li class="listitem"><p>
+ Input/Output
+ </p><p>
+ The destructor <code class="function">ios_base::Init::~Init()</code>
+ swallows all exceptions from <code class="function">flush</code> called on
+ all open streams at termination.
+ </p><p>
+ All formatted input in <code class="classname">basic_istream</code> or
+ formatted output in <code class="classname">basic_ostream</code> can be
+ configured to swallow exceptions
+ when <code class="function">exceptions</code> is set to
+ ignore <span class="type">ios_base::badbit</span>.
+ </p><p>
+ Functions that have been registered
+ with <code class="function">ios_base::register_callback</code> swallow all
+ exceptions when called as part of a callback event.
+ </p><p>
+ 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 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" 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
+ not impose hidden or unexpected costs. The GNU system tries to be
+ as flexible and as configurable as possible. So, it should come as
+ no surprise that GNU C++ provides an optional language extension,
+ spelled <code class="literal">-fno-exceptions</code>, as a way to excise the
+ implicitly generated magic necessary to
+ support <code class="literal">try</code> and <code class="literal">catch</code> blocks
+ and thrown objects. (Language support
+ for <code class="literal">-fno-exceptions</code> is documented in the GNU
+ GCC <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options" target="_top">manual</a>.)
+ </p><p>Before detailing the library support
+ for <code class="literal">-fno-exceptions</code>, first a passing note on
+ the things lost when this flag is used: it will break exceptions
+ trying to pass through code compiled
+ with <code class="literal">-fno-exceptions</code> whether or not that code
+ has any <code class="literal">try</code> or <code class="literal">catch</code>
+ constructs. If you might have some code that throws, you shouldn't
+ 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>
+ 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
+ capabilities of the underlying operating system and specific
+ configuration of the C++ compiler. On recent hardware with GNU
+ system software of the same age, the combined code and data size
+ overhead for enabling exception handling is around 7%. Of course,
+ if code size is of singular concern than using the appropriate
+ optimizer setting with exception handling enabled
+ (ie, <code class="literal">-Os -fexceptions</code>) may save up to twice
+ that, and preserve error checking.
+ </p><p>
+ So. Hell bent, we race down the slippery track, knowing the brakes
+ are a little soft and that the right front wheel has a tendency to
+ wobble at speed. Go on: detail the standard library support
+ for <code class="literal">-fno-exceptions</code>.
+ </p><p>
+ In sum, valid C++ code with exception handling is transformed into
+ a dialect without exception handling. In detailed steps: all use
+ of the C++
+ keywords <code class="literal">try</code>, <code class="literal">catch</code>,
+ and <code class="literal">throw</code> in the standard library have been
+ permanently replaced with the pre-processor controlled equivalents
+ spelled <code class="literal">__try</code>, <code class="literal">__catch</code>,
+ and <code class="literal">__throw_exception_again</code>. They are defined
+ as follows.
+ </p><pre class="programlisting">
+#ifdef __EXCEPTIONS
+# define __try try
+# define __catch(X) catch(X)
+# define __throw_exception_again throw
+#else
+# define __try if (true)
+# define __catch(X) if (false)
+# define __throw_exception_again
+#endif
+</pre><p>
+ In addition, for every object derived from
+ class <code class="classname">exception</code>, there exists a corresponding
+ function with C language linkage. An example:
+</p><pre class="programlisting">
+#ifdef __EXCEPTIONS
+ void __throw_bad_exception(void)
+ { throw bad_exception(); }
+#else
+ void __throw_bad_exception(void)
+ { abort(); }
+#endif
+</pre><p>
+ The last language feature needing to be transformed
+ by <code class="literal">-fno-exceptions</code> is treatment of exception
+ specifications on member functions. Fortunately, the compiler deals
+ with this by ignoring exception specifications and so no alternate
+ source markup is needed.
+</p><p>
+ By using this combination of language re-specification by the
+ compiler, and the pre-processor tricks and the functional
+ indirection layer for thrown exception objects by the library,
+ libstdc++ files can be compiled
+ with <code class="literal">-fno-exceptions</code>.
+</p><p>
+ User code that uses C++ keywords
+ like <code class="literal">throw</code>, <code class="literal">try</code>,
+ and <code class="literal">catch</code> will produce errors even if the user
+ code has included libstdc++ headers and is using constructs
+ like <code class="classname">basic_iostream</code>. Even though the standard
+ 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.
+</p><p>
+ Some issues remain with this approach (see bugzilla entry
+ 25191). Code paths are not equivalent, in
+ particular <code class="literal">catch</code> blocks are not evaluated. Also
+ problematic are <code class="literal">throw</code> expressions expecting a
+ user-defined throw handler. Known problem areas in the standard
+ library include using an instance
+ of <code class="classname">basic_istream</code>
+ with <code class="function">exceptions</code> set to specific
+ <span class="type">ios_base::iostate</span> conditions, or
+ cascading <code class="literal">catch</code> blocks that dispatch error
+ handling or recovery efforts based on the type of exception object
+ thrown.
+</p><p>
+ Oh, and by the way: none of this hackery is at all
+ special. (Although perhaps well-deserving of a raised eyebrow.)
+ Support continues to evolve and may change in the future. Similar
+ and even additional techniques are used in other C++ libraries and
+ compilers.
+</p><p>
+ C++ hackers with a bent for language and control-flow purity have
+ been successfully consoled by grizzled C veterans lamenting the
+ 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" 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
+ unwinding possible.
+</p><p>
+ In particular, unwinding into a frame with no exception handling
+data will cause a runtime abort. If the unwinder runs out of unwind
+info before it finds a handler, <code class="function">std::terminate()</code>
+is called.
+</p><p>
+ Please note that most development environments should take care of
+ 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" 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>
+ Cancellation points are functions defined by POSIX as worthy of
+ special treatment. The standard library may use some of these
+ functions to implement parts of the ISO C++ standard or depend on
+ them for extensions.
+</p><p>
+ Of note:
+</p><p>
+ <code class="function">nanosleep</code>,
+ <code class="function">read</code>, <code class="function">write</code>, <code class="function">open</code>, <code class="function">close</code>,
+ and <code class="function">wait</code>.
+</p><p>
+ The parts of libstdc++ that use C library functions marked as
+ cancellation points should take pains to be exception neutral.
+ Failing this, <code class="literal">catch</code> blocks have been augmented to
+ show that the POSIX cancellation object is in flight.
+</p><p>
+ This augmentation adds a <code class="literal">catch</code> block
+ for <code class="classname">__cxxabiv1::__forced_unwind</code>, which is the
+ object representing the POSIX cancellation object. Like so:
+</p><pre class="programlisting">
+ catch(const __cxxabiv1::__forced_unwind&)
+ {
+ this->_M_setstate(ios_base::badbit);
+ throw;
+ }
+ catch(...)
+ { this->_M_setstate(ios_base::badbit); }
+</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></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><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="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><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><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><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><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="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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