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 <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"><h3>Motivation</h3>
 </div>
 <p>Traditionally, when using exceptions to report failures, the throw site:</p>
 <div><ul><li>creates an exception object of the appropriate type, and</li>
 <li>stuffs it with data relevant to the detected error.</li>
 </ul></div>
 <p>A higher context in the program contains a catch statement which:</p>
 <div><ul><li>selects failures based on exception types, and</li>
 <li>inspects exception objects for data required to deal with the problem.</li>
 </ul></div>
 <p>The main issue with this "traditional" approach is that often, the data available at the point of the throw is insufficient for the catch site to handle the failure.</p>
 <p>Here is an example of a catch statement:</p>
 <pre>catch( file_read_error &amp; e )
     {
     std::cerr &lt;&lt; e.file_name();
     }</pre>
 <p>And here is a possible matching throw:</p>
 <pre>void
 read_file( FILE * f )
     {
     ....
     size_t nr=fread(buf,1,count,f);
     if( ferror(f) )
         throw file_read_error(???);
     ....
     }</pre>
 <p>Clearly, the problem is that the handler requires a file name but the read_file function does not have a file name to put in the exception object; all it has is a FILE pointer!</p>
 <p>In an attempt to deal with this problem, we could modify read_file to accept a file name:</p>
 <pre>void
 read_file( FILE * f, char const * name )
     {
     ....
     size_t nr=fread(buf,1,count,f);
     if( ferror(f) )
         throw file_read_error(name);
     ....
     }</pre>
 <p>This is not a real solution: it simply shifts the burden of supplying a file name to the immediate caller of the read_file function.</p>
 <blockquote><p><i>In general, the data required to handle a given library-emitted exception depends on the program that links to it. Many contexts between the throw and the catch may have relevant information which must be transported to the exception handler.</i></p></blockquote>
 <h3>Exception wrapping</h3>
 <p>The idea of exception wrapping is to catch an exception from a lower level function (such as the read_file function above), and throw a new exception object that contains the original exception (and also carries a file name.) This method seems to be particularly popular with C++ programmers with Java background.</p>
 <p>Exception wrapping leads to the following problems:</p>
 <div><ul><li>To wrap an exception object it must be copied, which may result in slicing.</li>
 <li>Wrapping is practically impossible to use in generic contexts.</li>
 </ul></div>
 <p>The second point is actually special case of violating the exception neutrality principle. Most contexts in a program can not handle exceptions; such contexts should not interfere with the process of exception handling.</p>
 <h3>The boost::exception solution</h3>
 <div><ul><li>Simply derive your exception types from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>.</li>
 <li>Confidently limit the throw site to provide only data that is available naturally.</li>
 <li>Use exception-neutral contexts between the throw and the catch to augment exceptions with more relevant data as they bubble up.</li>
 </ul></div>
 <p>For example, in the throw statement below we only add the errno code, since this is the only failure-relevant information available in this context:</p>
 <pre>struct exception_base: virtual std::exception, virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };
 struct io_error: virtual exception_base { };
 struct file_read_error: virtual io_error { };

 typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_errno_code,int&gt; errno_code;

 void
 read_file( FILE * f )
     {
     ....
     size_t nr=fread(buf,1,count,f);
     if( ferror(f) )
         throw file_read_error() <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> errno_code(errno);
     ....
     }</pre>
 <p>In a higher exception-neutral context, we add the file name to <i>any</i> exception that derives from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>:</p>
 <pre>typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span>&lt;struct tag_file_name,std::string&gt; file_name;

 ....
 try
     {
     if( FILE * fp=fopen("foo.txt","rt") )
         {
         shared_ptr&lt;FILE&gt; f(fp,fclose);
         ....
         read_file(fp); //throws types deriving from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>
         do_something();
         ....
         }
     else
         throw file_open_error() <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> errno_code(errno);
     }
 catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
     {
     e <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> file_name("foo.txt");
     throw;
     }</pre>
 <p>Finally here is how the handler retrieves data from exceptions that derive from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>:</p>
 <pre>catch( io_error &amp; e )
     {
     std::cerr &lt;&lt; "I/O Error!\n";

     if( std::string const * fn=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>&lt;file_name&gt;(e) )
         std::cerr &lt;&lt; "File name: " &lt;&lt; *fn &lt;&lt; "\n";

     if( int const * c=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>&lt;errno_code&gt;(e) )
         std::cerr &lt;&lt; "OS says: " &lt;&lt; strerror(*c) &lt;&lt; "\n";
     }</pre>
 <p>In addition, boost::<span class="RenoLink"><a href="diagnostic_information.html">diagnostic_information</a></span> can be used to compose an automatic (if not user-friendly) message that contains all of the <span class="RenoLink"><a href="error_info.html">error_info</a></span> objects added to a boost::<span class="RenoLink"><a href="exception.html">exception</a></span>. This is useful for inclusion in logs and other diagnostic objects.</p>
 </div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
 See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a>&nbsp;| <a href="exception_types_as_simple_semantic_tags.html">Exception Types as Simple Semantic Tags</a>&nbsp;| <a href="frequently_asked_questions.html">Frequently Asked Questions</a>&nbsp;| <a href="tutorial_enable_error_info.html">Integrating Boost Exception in Existing Exception Class Hierarchies</a></span>
 </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) -->
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 <small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/>
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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>Motivation</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="../../../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"><h3>Motivation</h3>
	</div>
	<p>Traditionally, when using exceptions to report failures, the throw site:</p>
	<div><ul><li>creates an exception object of the appropriate type, and</li>
	<li>stuffs it with data relevant to the detected error.</li>
	</ul></div>
	<p>A higher context in the program contains a catch statement which:</p>
	<div><ul><li>selects failures based on exception types, and</li>
	<li>inspects exception objects for data required to deal with the problem.</li>
	</ul></div>
	<p>The main issue with this "traditional" approach is that often, the data available at the point of the throw is insufficient for the catch site to handle the failure.</p>
	<p>Here is an example of a catch statement:</p>
	<pre>catch( file_read_error & e )
	{
	std::cerr << e.file_name();
	}</pre>
	<p>And here is a possible matching throw:</p>
	<pre>void
	read_file( FILE * f )
	{
	....
	size_t nr=fread(buf,1,count,f);
	if( ferror(f) )
	throw file_read_error(???);
	....
	}</pre>
	<p>Clearly, the problem is that the handler requires a file name but the read_file function does not have a file name to put in the exception object; all it has is a FILE pointer!</p>
	<p>In an attempt to deal with this problem, we could modify read_file to accept a file name:</p>
	<pre>void
	read_file( FILE * f, char const * name )
	{
	....
	size_t nr=fread(buf,1,count,f);
	if( ferror(f) )
	throw file_read_error(name);
	....
	}</pre>
	<p>This is not a real solution: it simply shifts the burden of supplying a file name to the immediate caller of the read_file function.</p>
	<blockquote><p><i>In general, the data required to handle a given library-emitted exception depends on the program that links to it. Many contexts between the throw and the catch may have relevant information which must be transported to the exception handler.</i></p></blockquote>
	<h3>Exception wrapping</h3>
	<p>The idea of exception wrapping is to catch an exception from a lower level function (such as the read_file function above), and throw a new exception object that contains the original exception (and also carries a file name.) This method seems to be particularly popular with C++ programmers with Java background.</p>
	<p>Exception wrapping leads to the following problems:</p>
	<div><ul><li>To wrap an exception object it must be copied, which may result in slicing.</li>
	<li>Wrapping is practically impossible to use in generic contexts.</li>
	</ul></div>
	<p>The second point is actually special case of violating the exception neutrality principle. Most contexts in a program can not handle exceptions; such contexts should not interfere with the process of exception handling.</p>
	<h3>The boost::exception solution</h3>
	<div><ul><li>Simply derive your exception types from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>.</li>
	<li>Confidently limit the throw site to provide only data that is available naturally.</li>
	<li>Use exception-neutral contexts between the throw and the catch to augment exceptions with more relevant data as they bubble up.</li>
	</ul></div>
	<p>For example, in the throw statement below we only add the errno code, since this is the only failure-relevant information available in this context:</p>
	<pre>struct exception_base: virtual std::exception, virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };
	struct io_error: virtual exception_base { };
	struct file_read_error: virtual io_error { };

	typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span><struct tag_errno_code,int> errno_code;

	void
	read_file( FILE * f )
	{
	....
	size_t nr=fread(buf,1,count,f);
	if( ferror(f) )
	throw file_read_error() <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> errno_code(errno);
	....
	}</pre>
	<p>In a higher exception-neutral context, we add the file name to <i>any</i> exception that derives from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>:</p>
	<pre>typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span><struct tag_file_name,std::string> file_name;

	....
	try
	{
	if( FILE * fp=fopen("foo.txt","rt") )
	{
	shared_ptr<FILE> f(fp,fclose);
	....
	read_file(fp); //throws types deriving from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>
	do_something();
	....
	}
	else
	throw file_open_error() <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> errno_code(errno);
	}
	catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> & e )
	{
	e <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> file_name("foo.txt");
	throw;
	}</pre>
	<p>Finally here is how the handler retrieves data from exceptions that derive from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>:</p>
	<pre>catch( io_error & e )
	{
	std::cerr << "I/O Error!\n";

	if( std::string const * fn=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span><file_name>(e) )
	std::cerr << "File name: " << *fn << "\n";

	if( int const * c=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span><errno_code>(e) )
	std::cerr << "OS says: " << strerror(*c) << "\n";
	}</pre>
	<p>In addition, boost::<span class="RenoLink"><a href="diagnostic_information.html">diagnostic_information</a></span> can be used to compose an automatic (if not user-friendly) message that contains all of the <span class="RenoLink"><a href="error_info.html">error_info</a></span> objects added to a boost::<span class="RenoLink"><a href="exception.html">exception</a></span>. This is useful for inclusion in logs and other diagnostic objects.</p>
	</div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
	See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a> \| <a href="exception_types_as_simple_semantic_tags.html">Exception Types as Simple Semantic Tags</a> \| <a href="frequently_asked_questions.html">Frequently Asked Questions</a> \| <a href="tutorial_enable_error_info.html">Integrating Boost Exception in Existing Exception Class Hierarchies</a></span>
	</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) -->
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	<small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/>
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