diff options
Diffstat (limited to 'libstdc++-v3/doc/html/manual/using_exceptions.html')
| -rw-r--r-- | libstdc++-v3/doc/html/manual/using_exceptions.html | 307 |
1 files changed, 307 insertions, 0 deletions
diff --git a/libstdc++-v3/doc/html/manual/using_exceptions.html b/libstdc++-v3/doc/html/manual/using_exceptions.html new file mode 100644 index 000000000..d1e2693ab --- /dev/null +++ b/libstdc++-v3/doc/html/manual/using_exceptions.html @@ -0,0 +1,307 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Exceptions</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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"/><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 align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr></table><hr/></div><div class="section" title="Exceptions"><div class="titlepage"><div><div><h2 class="title"><a id="manual.intro.using.exceptions"/>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="section" title="Exception Safety"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.safety"/>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"><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="section" title="Exception Neutrality"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.propagating"/>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"><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="section" title="Doing without"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"/>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="link" href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options">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="section" title="Compatibility"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.compat"/>Compatibility</h3></div></div></div><div class="section" title="With C"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.c"/>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="section" title="With POSIX thread cancellation"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.posix"/>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"/>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id470759"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008) + </em>. </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="id470792"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + Error and Exception Handling + </em>. </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="id470825"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + Exception-Safety in Generic Components + </em>. </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="id470858"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + Standard Library Exception Policy + </em>. </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="id470891"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + ia64 c++ abi exception handling + </em>. </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="id470924"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + Appendix E: Standard-Library Exception Safety + </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span></p></div><div class="biblioentry"><a id="id470950"/><p><span class="citetitle"><em class="citetitle"> + Exceptional C++ + </em>. </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="id470970"/><p><span class="biblioid"> + . </span><span class="citetitle"><em class="citetitle"> + GCC Bug 25191: exception_defines.h #defines try/catch + </em>. </span></p></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><td align="center"><a accesskey="u" href="using.html">Up</a></td><td align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr><tr><td align="left" valign="top">Concurrency </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Debugging Support</td></tr></table></div></body></html> |