1<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN' 2'http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd'> 3<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'> 4<head> 5 <meta http-equiv='Content-Type' content='text/html; charset=utf-8'/> 6 <title>transporting of exceptions between threads</title> 7 <link href='reno.css' type='text/css' rel='stylesheet'/> 8</head> 9<body> 10<div class="body-0"> 11<div class="body-1"> 12<div class="body-2"> 13<div> 14<div id="boost_logo"> 15<a href="http://www.boost.org"><img style="border:0" src="../../../boost.png" alt="Boost" width="277" height="86"/></a> 16</div> 17<h1>Boost Exception</h1> 18</div> 19<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. --> 20<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying --> 21<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 22<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h2>Transporting of Exceptions Between Threads</h2> 23</div> 24<p>Boost Exception supports transporting of exception objects between threads through cloning. This system is similar to <span class="RenoLink"><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2179.html">N2179</a></span>, but because Boost Exception can not rely on language support, the use of <span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span> at the time of the throw is required in order to use cloning.</p> 25<h4>Note:</h4> 26<p>All exceptions emitted by the familiar function boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span> are guaranteed to derive from boost::<span class="RenoLink"><a href="exception.html">exception</a></span> and to support cloning.</p> 27<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Using enable_current_exception at the Time of the Throw</h3> 28</div> 29<p>Here is how cloning can be enabled in a throw-expression (15.1):</p> 30<pre>#include <<span class="RenoLink"><a href="boost_exception_info_hpp.html">boost/exception/info.hpp</a></span>> 31#include <<span class="RenoLink"><a href="boost_exception_errinfo_errno_hpp.html">boost/exception/errinfo_errno.hpp</a></span>> 32#include <stdio.h> 33#include <errno.h> 34 35struct file_read_error: virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { }; 36 37void 38file_read( FILE * f, void * buffer, size_t size ) 39 { 40 if( size!=fread(buffer,1,size,f) ) 41 throw boost::<span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>(file_read_error()) << 42 boost::<span class="RenoLink"><a href="errinfo_errno.html">errinfo_errno</a></span>(errno); 43 }</pre> 44<p>Of course, <span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span> may be used with any exception type; there is no requirement that it should derive from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>.</p> 45</div><div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Cloning and Re-Throwing an Exception</h3> 46</div> 47<p>When you catch an exception, you can call <span class="RenoLink"><a href="current_exception.html">current_exception</a></span> to get an <span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> object:</p> 48<pre>#include <<span class="RenoLink"><a href="boost_exception_ptr_hpp.html">boost/exception_ptr.hpp</a></span>> 49#include <boost/thread.hpp> 50#include <boost/bind.hpp> 51 52void do_work(); //throws cloning-enabled boost::<span class="RenoLink"><a href="exception.html">exception</a></span>s 53 54void 55worker_thread( boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> & error ) 56 { 57 try 58 { 59 do_work(); 60 error = boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span>(); 61 } 62 catch( 63 ... ) 64 { 65 error = boost::<span class="RenoLink"><a href="current_exception.html">current_exception</a></span>(); 66 } 67 }</pre> 68<p>In the above example, note that <span class="RenoLink"><a href="current_exception.html">current_exception</a></span> captures the original type of the exception object. The exception can be thrown again using the <span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span> function:</p> 69<pre>// ...continued 70 71void 72work() 73 { 74 boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> error; 75 boost::<span class="RenoLink"><a href="http://www.boost.org/doc/html/boost/thread.html">thread</a></span> t( boost::<span class="RenoLink"><a href="http://www.boost.org/libs/bind/bind.html">bind</a></span>(worker_thread,boost::<span class="RenoLink"><a href="http://www.boost.org/doc/html/ref.html">ref</a></span>(error)) ); 76 t.<span class="RenoLink"><a href="http://www.boost.org/doc/html/boost/thread.html">join</a></span>(); 77 if( error ) 78 boost::<span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>(error); 79 }</pre> 80<p>Note that <span class="RenoLink"><a href="current_exception.html">current_exception</a></span> could fail to copy the original exception object in the following cases:</p> 81<div><ul><li> if there is not enough memory, in which case the returned <span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> points to an instance of std::bad_alloc, or</li> 82<li> if <span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span> was not used in the throw-expression passed to the original throw statement and the current implementation does not have the necessary compiler-specific support to copy the exception automatically, in which case the returned <span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> points to an instance of <span class="RenoLink"><a href="unknown_exception.html">unknown_exception</a></span>.</li> 83</ul></div> 84<p>Regardless, the use of <span class="RenoLink"><a href="current_exception.html">current_exception</a></span> and <span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span> in the above examples is well-formed.</p> 85</div></div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div> 86See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a></span> 87</div> 88<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. --> 89<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying --> 90<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 91<div id="footer"> 92<p> 93<a class="logo" href="http://jigsaw.w3.org/css-validator/check/referer"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a> 94<a class="logo" href="http://validator.w3.org/check?uri=referer"><img class="logo_pic" src="valid-xhtml.png" alt="Valid XHTML 1.0" height="31" width="88"/></a> 95<small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/> 96Distributed under the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version 1.0</a>.</small> 97</p> 98</div> 99</div> 100</div> 101</div> 102</body> 103</html> 104
