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<h1>Boost Exception</h1>
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<div class="RenoIncludeDIV"><h2>Transporting of Exceptions between Threads</h2>
<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 <tt><span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>()</tt> 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 <tt>boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span>()</tt> are guaranteed to derive from <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt> and to support cloning.</p>
<div class="RenoIncludeDIV"><h3>Using enable_current_exception() at the Time of the Throw</h3>
<p>Here is how cloning can be enabled in a throw-expression (15.1):</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_enable_exception_cloning_hpp.html">boost/exception/enable_current_exception.hpp</a></span>&gt;
#include &lt;<span class="RenoLink"><a href="exception_error_info_hpp.html">boost/exception/info.hpp</a></span>&gt;
#include &lt;stdio.h&gt;
#include &lt;errno.h&gt;

typedef boost::error_info&lt;struct tag_errno,int&gt; errno_info;

class file_read_error: public 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;
            errno_info(errno);
    }</pre>
</div><div class="RenoIncludeDIV"><h3>Cloning and Re-throwing an Exception</h3>
<p>When you catch a <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>, you can call <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> to get an <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> object:</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_cloning_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 <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> captures the original type of the exception object, even though <tt>e</tt> refers to the base type <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>. This original type can be thrown again using the <tt><span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>()</tt> 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><tt><span class="RenoLink"><a href="current_exception.html">Clone_exception</a></span>()</tt> 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 <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> points to an instance of <tt>std::bad_alloc</tt>, or</li>
<li> if <tt><span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>()</tt> was not used in the throw-expression passed to the original <tt>throw</tt> statement, in which case the returned <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> points to an instance of <tt><span class="RenoLink"><a href="unknown_exception.html">unknown_exception</a></span></tt>.</li>
</ul></div>
<p>Regardless, the use of <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> and <tt><span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>()</tt> in the above examples is well-formed.</p>
</div></div><h3>See also:</h3>
<div class="RenoPageList"><a href="boost-exception.html">Boost Exception<br/>
</a><a href="unknown_exception.html">unknown_exception<br/>
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boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'`
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			`<head>`
			`<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>`
			`<title>Tutorial: Transporting of Exceptions between Threads</title>`
			`<link href='reno.css' type='text/css' rel='stylesheet'/>`
			`</head>`
			`<body>`
			`<div class="body-0">`
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			`</div>`
			`<h1>Boost Exception</h1>`
			`</div>`
			`<div class="RenoIncludeDIV"><h2>Transporting of Exceptions between Threads</h2>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<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 <tt><span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>()</tt> at the time of the throw is required in order to use cloning.</p>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`<h4>Note:</h4>`
			`<p>All exceptions emitted by the familiar function <tt>boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span>()</tt> are guaranteed to derive from <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt> and to support cloning.</p>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<div class="RenoIncludeDIV"><h3>Using enable_current_exception() at the Time of the Throw</h3>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`<p>Here is how cloning can be enabled in a throw-expression (15.1):</p>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<pre>#include <<span class="RenoLink"><a href="exception_enable_exception_cloning_hpp.html">boost/exception/enable_current_exception.hpp</a></span>>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`#include <<span class="RenoLink"><a href="exception_error_info_hpp.html">boost/exception/info.hpp</a></span>>`
			`#include <stdio.h>`
			`#include <errno.h>`

			`typedef boost::error_info<struct tag_errno,int> errno_info;`

			`class file_read_error: public 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) )`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`throw boost::<span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>(file_read_error()) <<`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`errno_info(errno);`
			`}</pre>`
			`</div><div class="RenoIncludeDIV"><h3>Cloning and Re-throwing an Exception</h3>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<p>When you catch a <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>, you can call <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> to get an <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> object:</p>`
			`<pre>#include <<span class="RenoLink"><a href="exception_cloning_hpp.html">boost/exception_ptr.hpp</a></span>>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`#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(`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`... )`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`{`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`error = boost::<span class="RenoLink"><a href="current_exception.html">current_exception</a></span>();`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`}`
			`}</pre>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<p>In the above example, note that <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> captures the original type of the exception object, even though <tt>e</tt> refers to the base type <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>. This original type can be thrown again using the <tt><span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>()</tt> function:</p>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`<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>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<p><tt><span class="RenoLink"><a href="current_exception.html">Clone_exception</a></span>()</tt> could fail to copy the original exception object in the following cases:</p>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`<div><ul><li> if there is not enough memory, in which case the returned <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> points to an instance of <tt>std::bad_alloc</tt>, or</li>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<li> if <tt><span class="RenoLink"><a href="enable_current_exception.html">enable_current_exception</a></span>()</tt> was not used in the throw-expression passed to the original <tt>throw</tt> statement, in which case the returned <tt><span class="RenoLink"><a href="exception_ptr.html">exception_ptr</a></span></tt> points to an instance of <tt><span class="RenoLink"><a href="unknown_exception.html">unknown_exception</a></span></tt>.</li>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`</ul></div>`
Boost Exception documentation update [SVN r44444] 2008-04-15 21:56:34 +00:00			`<p>Regardless, the use of <tt><span class="RenoLink"><a href="current_exception.html">current_exception</a></span>()</tt> and <tt><span class="RenoLink"><a href="rethrow_exception.html">rethrow_exception</a></span>()</tt> in the above examples is well-formed.</p>`
boost exception [SVN r43485] 2008-03-04 01:41:17 +00:00			`</div></div><h3>See also:</h3>`
			`<div class="RenoPageList"><a href="boost-exception.html">Boost Exception<br/>`
			`</a><a href="unknown_exception.html">unknown_exception<br/>`
			`</a></div>`
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