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Emil Dotchevski
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<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
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<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
<title>Tutorial: Adding of Arbitrary Data at the Point of the Throw</title>
<link href='reno.css' type='text/css' rel='stylesheet'/>
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<div id="boost_logo">
<a href="http://www.boost.org"><img style="border:0" src="http://www.boost.org/boost.png" alt="Boost" width="277" height="86"/></a>
</div>
<h1>Boost Exception</h1>
</div>
<div class="RenoIncludeDIV"><h3>Adding of Arbitrary Data at the Point of the Throw</h3>
<p>The following example demonstrates how <tt>errno</tt> can be stored in exception objects using Boost Exception:</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_hpp.html">boost/exception.hpp</a></span>&gt;
#include &lt;errno.h&gt;
#include &lt;iostream&gt;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_errno,int&gt; errno_info; //(1)
class my_error: public boost::<span class="RenoLink"><a href="exception.html">exception</a></span>, public std::exception { }; //(2)
void
f()
{
throw my_error() &lt;&lt; errno_info(errno); //(3)
}
</pre>
<p>First, we instantiate the <tt><span class="RenoLink"><a href="error_info.html">error_info</a></span></tt> template using a unique identifier -- <tt>tag_errno</tt>, and the type of the info it identifies -- <tt>int</tt>. This provides compile-time type safety for the various values stored in exception objects.</p>
<p>Second, we define class <tt>my_error</tt>, which derives from <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>.</p>
<p>Finally, (3) illustrates how the <tt>typedef</tt> from (1) can be used with <tt><span class="RenoLink"><a href="operator_shl_exception.html">operator&lt;&lt;</a></span>()</tt> to store values in exception objects at the point of the throw.</p>
<p>The stored <tt>errno</tt> value can be recovered at a later time like this:</p>
<pre>// ...continued
void
g()
{
try
{
f();
}
catch(
my_error &amp; x )
{
if( boost::shared_ptr&lt;int const&gt; err=boost::<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>&lt;errno_info&gt;(x) )
std::cerr &lt;&lt; "Error code: " &lt;&lt; *err;
}
}</pre>
<p>The <tt><span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>()</tt> function template is instantiated with the <tt>typedef</tt> from (1), and is passed an exception object of any type that derives publicly from <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>. If the exception object contains the requested value, the returned <tt><span class="RenoLink"><a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm">shared_ptr</a></span></tt> will point to it; otherwise an empty <tt><span class="RenoLink"><a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm">shared_ptr</a></span></tt> is returned.</p>
</div><h3>See also:</h3>
<div class="RenoPageList"><a href="transporting_data.html">Tutorial: Transporting of Arbitrary Data to the Catch Site<br/>
</a></div>
<div id="footer">
<p>&nbsp;</p>
<hr/>
<p>
<a class="logo" href="http://jigsaw.w3.org/css-validator/validator?uri=http://revergestudios.com/boost-exception/reno.css"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a>
<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>
<small>Copyright (c) 2006-2008 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>
</p>
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<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
'http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd'>
<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
<title>Tutorial: Adding of Arbitrary Data to Active Exception Objects</title>
<link href='reno.css' type='text/css' rel='stylesheet'/>
</head>
<body>
<div class="body-0">
<div class="body-1">
<div class="body-2">
<div>
<div id="boost_logo">
<a href="http://www.boost.org"><img style="border:0" src="http://www.boost.org/boost.png" alt="Boost" width="277" height="86"/></a>
</div>
<h1>Boost Exception</h1>
</div>
<div class="RenoIncludeDIV"><h3>Adding of Arbitrary Data to Active Exception Objects</h3>
<p>Sometimes the throw site does not have all the information that is needed at the catch site to make sense of what went wrong. Here is an example:</p>
<pre>#include &lt;stdio.h&gt;
#include &lt;string&gt;
class
file_read_error
{
public:
explicit
file_read_error( std::string const &amp; fn ):
fn_(fn)
{
};
std::string const &amp;
file_name() const
{
return fn_;
}
private:
std::string fn_;
};
void
file_read( FILE * f, void * buffer, size_t size )
{
if( size!=fread(buffer,1,size,f) )
throw file_read_error("????");
}</pre>
<p>We have defined an exception class <tt>file_read_error</tt> which can store a file name, so that when we catch a <tt>file_read_error</tt> object, we know which file the failure is related to. However, the <tt>file_read</tt> function does not have the file name at the time of the throw; all it has is a <tt>FILE</tt> handle.</p>
<p>One possible solution is to not use <tt>FILE</tt> handles directly. We could have our own <tt>class file</tt> which stores both a <tt>FILE</tt> handle and a file name, and pass that to <tt>file_read()</tt>. However, this could be problematic if we communicate with 3rd party code that does not use our <tt>class file</tt> (probably because they have their own similar class.)</p>
<p>A better solution is to make class <tt>file_read_error</tt> derive (possibly indirectly) from <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt>, and free the <tt>file_read()</tt> function from the burden of storing the file name in exceptions it throws:</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_hpp.html">boost/exception.hpp</a></span>&gt;
#include &lt;stdio.h&gt;
#include &lt;errno.h&gt;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&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 file_read_error() &lt;&lt; errno_info(errno);
}</pre>
<p>If <tt>file_read()</tt> detects a failure, it throws an exception which contains the information that is available at the time, namely the <tt>errno</tt>. Other relevant information, such as the file name, can be added in a context higher up the call stack, where it is known naturally:</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_hpp.html">boost/exception.hpp</a></span>&gt;
#include &lt;boost/shared_ptr.hpp&gt;
#include &lt;stdio.h&gt;
#include &lt;string&gt;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_file_name,std::string&gt; file_name_info;
boost::shared_ptr&lt;FILE&gt; file_open( char const * file_name, char const * mode );
void file_read( FILE * f, void * buffer, size_t size );
void
parse_file( char const * file_name )
{
boost::shared_ptr&lt;FILE&gt; f = file_open(file_name,"rb");
assert(f);
try
{
char buf[1024];
file_read( f.get(), buf, sizeof(buf) );
}
catch(
boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
{
e &lt;&lt; file_name_info(file_name);
throw;
}
}</pre>
<p>The above function is (almost) exception-neutral -- if an exception is emitted by any function call within the <tt>try</tt> block, <tt>parse_file()</tt> does not need to do any real work, but it intercepts any <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt> object, stores the file name, and re-throws using a throw-expression with no operand (15.1.6). The rationale for catching any <tt>boost::<span class="RenoLink"><a href="exception.html">exception</a></span></tt> object is that the file name is relevant to any failure that occurs in <tt>parse_file()</tt>, <i>even if the failure is unrelated to file I/O</i>.</p>
<p>As usual, the stored data can be retrieved using <tt><span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>()</tt>.</p>
</div><h3>See also:</h3>
<div class="RenoPageList"><a href="transporting_data.html">Tutorial: Transporting of Arbitrary Data to the Catch Site<br/>
</a></div>
<div id="footer">
<p>&nbsp;</p>
<hr/>
<p>
<a class="logo" href="http://jigsaw.w3.org/css-validator/validator?uri=http://revergestudios.com/boost-exception/reno.css"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a>
<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>
<small>Copyright (c) 2006-2008 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>
</p>
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<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
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<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
<title>Tutorial: Cloning and Re-throwing an Exception</title>
<link href='reno.css' type='text/css' rel='stylesheet'/>
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<body>
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<a href="http://www.boost.org"><img style="border:0" src="http://www.boost.org/boost.png" alt="Boost" width="277" height="86"/></a>
</div>
<h1>Boost Exception</h1>
</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="clone_exception.html">clone_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/cloning.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(
boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
{
error = boost::<span class="RenoLink"><a href="clone_exception.html">clone_exception</a></span>(e);
}
}</pre>
<p>In the above example, note that <tt><span class="RenoLink"><a href="clone_exception.html">clone_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="clone_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_exception_cloning.html">enable_exception_cloning</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="clone_exception.html">clone_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><h3>See also:</h3>
<div class="RenoPageList"><a href="cloning.html">Tutorial: Transporting of Exceptions between Threads<br/>
</a></div>
<div id="footer">
<p>&nbsp;</p>
<hr/>
<p>
<a class="logo" href="http://jigsaw.w3.org/css-validator/validator?uri=http://revergestudios.com/boost-exception/reno.css"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a>
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<small>Copyright (c) 2006-2008 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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<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
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<title>Tutorial: Adding Grouped Data to Exceptions</title>
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</div>
<h1>Boost Exception</h1>
</div>
<div class="RenoIncludeDIV"><h3>Adding Grouped Data to Exceptions</h3>
<p>The code snippet below demonstrates how <tt>boost::<span class="RenoLink"><a href="http://www.boost.org/libs/tuple/doc/tuple_users_guide.html">tuple</a></span></tt> can be used to bundle the name of the function that failed, together with the reported <tt>errno</tt> so that they can be added to exception objects more conveniently together:</p>
<pre>#include &lt;<span class="RenoLink"><a href="exception_error_info_group_hpp.html">boost/exception/info_tuple.hpp</a></span>&gt;
#include &lt;boost/shared_ptr.hpp&gt;
#include &lt;stdio.h&gt;
#include &lt;string&gt;
#include &lt;errno&gt;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_file_name,std::string&gt; file_name_info;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_function,char const *&gt; function_info;
typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_errno,int&gt; errno_info;
typedef boost::tuple&lt;function_info,errno_info&gt; clib_failure;
class file_open_error: public boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };
boost::shared_ptr&lt;FILE&gt;
file_open( char const * name, char const * mode )
{
if( FILE * f=fopen(name,mode) )
return boost::shared_ptr&lt;FILE&gt;(f,fclose);
else
throw file_open_error() &lt;&lt;
file_name_info(name) &lt;&lt;
clib_failure("fopen",errno);
}</pre>
<p>Note that the members of a <tt>boost::<span class="RenoLink"><a href="http://www.boost.org/libs/tuple/doc/tuple_users_guide.html">tuple</a></span></tt> are stored separately in exception objects; they can only be retrieved individually, using <tt><span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>()</tt>.</p>
</div><h3>See also:</h3>
<div class="RenoPageList"><a href="transporting_data.html">Tutorial: Transporting of Arbitrary Data to the Catch Site<br/>
</a></div>
<div id="footer">
<p>&nbsp;</p>
<hr/>
<p>
<a class="logo" href="http://jigsaw.w3.org/css-validator/validator?uri=http://revergestudios.com/boost-exception/reno.css"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a>
<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>
<small>Copyright (c) 2006-2008 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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/*
* Copyright (c) 2006-2008 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)
*/
body
{
font-family: Trebuchet, Verdana, Arial, Helvetica, Sans;

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<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
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<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
<title>Tutorial: Using enable_exception_cloning() at the Time of the Throw</title>
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</head>
<body>
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<a href="http://www.boost.org"><img style="border:0" src="http://www.boost.org/boost.png" alt="Boost" width="277" height="86"/></a>
</div>
<h1>Boost Exception</h1>
</div>
<div class="RenoIncludeDIV"><h3>Using enable_exception_cloning() 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_exception_cloning.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_exception_cloning.html">enable_exception_cloning</a></span>(file_read_error()) &lt;&lt;
errno_info(errno);
}</pre>
</div><h3>See also:</h3>
<div class="RenoPageList"><a href="cloning.html">Tutorial: Transporting of Exceptions between Threads<br/>
</a></div>
<div id="footer">
<p>&nbsp;</p>
<hr/>
<p>
<a class="logo" href="http://jigsaw.w3.org/css-validator/validator?uri=http://revergestudios.com/boost-exception/reno.css"><img class="logo_pic" src="valid-css.png" alt="Valid CSS" height="31" width="88"/></a>
<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>
<small>Copyright (c) 2006-2008 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>
</p>
</div>
</div>
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