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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"><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>
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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>
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<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
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7c673cae FG	1	<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Strict//EN'
	2	'http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd'>
	3	<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
	4	<head>
	5	<meta http-equiv='Content-Type' content='text/html; charset=utf-8'/>
	6	<title>Motivation</title>
	7	<link href='reno.css' type='text/css' rel='stylesheet'/>
	8	</head>
	9	<body>
	10	<div class="body-0">
	11	<div class="body-1">
	12	<div class="body-2">
	13	<div>
	14	<div id="boost_logo">
	15	<a href="http://www.boost.org"><img style="border:0" src="../../../boost.png" alt="Boost" width="277" height="86"/></a>
	16	</div>
	17	<h1>Boost Exception</h1>
	18	</div>
	19	<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
	20	<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
	21	<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
	22	<div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Motivation</h3>
	23	</div>
	24	<p>Traditionally, when using exceptions to report failures, the throw site:</p>
	25	<div><ul><li>creates an exception object of the appropriate type, and</li>
	26	<li>stuffs it with data relevant to the detected error.</li>
	27	</ul></div>
	28	<p>A higher context in the program contains a catch statement which:</p>
	29	<div><ul><li>selects failures based on exception types, and</li>
	30	<li>inspects exception objects for data required to deal with the problem.</li>
	31	</ul></div>
	32	<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>
	33	<p>Here is an example of a catch statement:</p>
	34	<pre>catch( file_read_error & e )
	35	{
	36	std::cerr << e.file_name();
	37	}</pre>
	38	<p>And here is a possible matching throw:</p>
	39	<pre>void
	40	read_file( FILE * f )
	41	{
	42	....
	43	size_t nr=fread(buf,1,count,f);
	44	if( ferror(f) )
	45	throw file_read_error(???);
	46	....
	47	}</pre>
	48	<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>
	49	<p>In an attempt to deal with this problem, we could modify read_file to accept a file name:</p>
	50	<pre>void
	51	read_file( FILE * f, char const * name )
	52	{
	53	....
	54	size_t nr=fread(buf,1,count,f);
	55	if( ferror(f) )
	56	throw file_read_error(name);
	57	....
	58	}</pre>
	59	<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>
	60	<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>
	61	<h3>Exception wrapping</h3>
	62	<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>
	63	<p>Exception wrapping leads to the following problems:</p>
	64	<div><ul><li>To wrap an exception object it must be copied, which may result in slicing.</li>
65	<li>Wrapping is practically impossible to use in generic contexts.</li>
66	</ul></div>
67	<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>
68	<h3>The boost::exception solution</h3>
69	<div><ul><li>Simply derive your exception types from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>.</li>
70	<li>Confidently limit the throw site to provide only data that is available naturally.</li>
71	<li>Use exception-neutral contexts between the throw and the catch to augment exceptions with more relevant data as they bubble up.</li>
72	</ul></div>
73	<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>
74	<pre>struct exception_base: virtual std::exception, virtual boost::<span class="RenoLink"><a href="exception.html">exception</a></span> { };
75	struct io_error: virtual exception_base { };
76	struct file_read_error: virtual io_error { };
77
78	typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span><struct tag_errno_code,int> errno_code;
79
80	void
81	read_file( FILE * f )
82	{
83	....
84	size_t nr=fread(buf,1,count,f);
85	if( ferror(f) )
86	throw file_read_error() <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> errno_code(errno);
87	....
88	}</pre>
89	<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>
90	<pre>typedef boost::<span class="RenoLink"><a href="error_info.html">error_info</a></span><struct tag_file_name,std::string> file_name;
91
92	....
93	try
94	{
95	if( FILE * fp=fopen("foo.txt","rt") )
96	{
97	shared_ptr<FILE> f(fp,fclose);
98	....
99	read_file(fp); //throws types deriving from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>
100	do_something();
101	....
102	}
103	else
104	throw file_open_error() <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> errno_code(errno);
105	}
106	catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> & e )
107	{
108	e <span class="RenoLink"><a href="exception_operator_shl.html"><<</a></span> file_name("foo.txt");
109	throw;
110	}</pre>
111	<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>
112	<pre>catch( io_error & e )
113	{
114	std::cerr << "I/O Error!\n";
115
116	if( std::string const * fn=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span><file_name>(e) )
117	std::cerr << "File name: " << *fn << "\n";
118
119	if( int const * c=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span><errno_code>(e) )
120	std::cerr << "OS says: " << strerror(*c) << "\n";
121	}</pre>
122	<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>
123	</div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
124	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>
125	</div>
126	<!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
127	<!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
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133	<small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/>
134	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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