boost_1_45_0/libs/exception/doc/tutorial_exception_ptr.html - nest-learning-thermostat/5.0/boost - Git at Google

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 <h1>Boost Exception</h1>
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 <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
 <!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
 <div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h2>Transporting of Exceptions Between Threads</h2>
 </div>
 <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>
 <h4>Note:</h4>
 <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>
 <div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Using enable_current_exception at the Time of the Throw</h3>
 </div>
 <p>Here is how cloning can be enabled in a throw-expression (15.1):</p>
 <pre>#include &lt;<span class="RenoLink"><a href="boost_exception_info_hpp.html">boost/exception/info.hpp</a></span>&gt;
 #include &lt;<span class="RenoLink"><a href="boost_exception_errinfo_errno_hpp.html">boost/exception/errinfo_errno.hpp</a></span>&gt;
 #include &lt;stdio.h&gt;
 #include &lt;errno.h&gt;

 struct file_read_error: virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };

 void
 file_read( FILE * f, void * buffer, size_t size )
     {
     if( size!=fread(buffer,1,size,f) )
         throw boost::<span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>(file_read_error()) &lt;&lt;
             boost::<span class="RenoLink"><a href="errinfo_errno.html">errinfo_errno</a></span>(errno);
     }</pre>
 <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>
 </div><div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Cloning and Re-Throwing an Exception</h3>
 </div>
 <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>
 <pre>#include &lt;<span class="RenoLink"><a href="boost_exception_ptr_hpp.html">boost/exception_ptr.hpp</a></span>&gt;
 #include &lt;boost/thread.hpp&gt;
 #include &lt;boost/bind.hpp&gt;

 void do_work(); //throws cloning-enabled boost::<span class="RenoLink"><a href="exception.html">exception</a></span>s

 void
 worker_thread( boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> &amp; error )
     {
     try
         {
         do_work();
         error = boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span>();
         }
     catch(
     ... )
         {
         error = boost::<span class="RenoLink"><a href="current_exception.html">current_exception</a></span>();
         }
     }</pre>
 <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>
 <pre>// ...continued

 void
 work()
     {
     boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> error;
     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)) );
     t.<span class="RenoLink"><a href="http://www.boost.org/doc/html/boost/thread.html">join</a></span>();
     if( error )
         boost::<span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>(error);
     }</pre>
 <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>
 <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>
 <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>
 </ul></div>
 <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>
 </div></div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
 See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a></span>
 </div>
 <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
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 <small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/>
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	<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
	<title>transporting of exceptions between threads</title>
	<link href='reno.css' type='text/css' rel='stylesheet'/>
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	<div class="body-0">
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	<div id="boost_logo">
	<a href="http://www.boost.org"><img style="border:0" src="../../../boost.png" alt="Boost" width="277" height="86"/></a>
	</div>
	<h1>Boost Exception</h1>
	</div>
	<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
	<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
	<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
	<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h2>Transporting of Exceptions Between Threads</h2>
	</div>
	<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>
	<h4>Note:</h4>
	<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>
	<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Using enable_current_exception at the Time of the Throw</h3>
	</div>
	<p>Here is how cloning can be enabled in a throw-expression (15.1):</p>
	<pre>#include <<span class="RenoLink"><a href="boost_exception_info_hpp.html">boost/exception/info.hpp</a></span>>
	#include <<span class="RenoLink"><a href="boost_exception_errinfo_errno_hpp.html">boost/exception/errinfo_errno.hpp</a></span>>
	#include <stdio.h>
	#include <errno.h>

	struct file_read_error: virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };

	void
	file_read( FILE * f, void * buffer, size_t size )
	{
	if( size!=fread(buffer,1,size,f) )
	throw boost::<span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>(file_read_error()) <<
	boost::<span class="RenoLink"><a href="errinfo_errno.html">errinfo_errno</a></span>(errno);
	}</pre>
	<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>
	</div><div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Cloning and Re-Throwing an Exception</h3>
	</div>
	<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>
	<pre>#include <<span class="RenoLink"><a href="boost_exception_ptr_hpp.html">boost/exception_ptr.hpp</a></span>>
	#include <boost/thread.hpp>
	#include <boost/bind.hpp>

	void do_work(); //throws cloning-enabled boost::<span class="RenoLink"><a href="exception.html">exception</a></span>s

	void
	worker_thread( boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> & error )
	{
	try
	{
	do_work();
	error = boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span>();
	}
	catch(
	... )
	{
	error = boost::<span class="RenoLink"><a href="current_exception.html">current_exception</a></span>();
	}
	}</pre>
	<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>
	<pre>// ...continued

	void
	work()
	{
	boost::<span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span> error;
	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)) );
	t.<span class="RenoLink"><a href="http://www.boost.org/doc/html/boost/thread.html">join</a></span>();
	if( error )
	boost::<span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>(error);
	}</pre>
	<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>
	<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>
	<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>
	</ul></div>
	<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>
	</div></div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
	See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a></span>
	</div>
	<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
	<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
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	<small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/>
	Distributed under the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version 1.0</a>.</small>
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