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<h1>Boost Exception</h1>
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<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
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<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h2>Transporting of Exceptions Between Threads</h2>
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<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>
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<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>
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<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>
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See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a></span>
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