Boost_1_39_0/libs/exception/doc/motivation.html

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  17 <h1>Boost Exception</h1>
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  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 &amp; e )
  35     {
  36     std::cerr &lt;&lt; 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>&lt;struct tag_errno_code,int&gt; 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">&lt;&lt;</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>&lt;struct tag_file_name,std::string&gt; file_name;
  91
  92 ....
  93 try
  94     {
  95     if( FILE * fp=fopen("foo.txt","rt") )
  96         {
  97         shared_ptr&lt;FILE&gt; 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">&lt;&lt;</a></span> errno_code(errno);
 105     }
 106 catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
 107     {
 108     e <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</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 &amp; e )
 113     {
 114     std::cerr &lt;&lt; "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>&lt;file_name&gt;(e) )
 117         std::cerr &lt;&lt; "File name: " &lt;&lt; *fn &lt;&lt; "\n";
 118
 119     if( int const * c=<span class="RenoLink"><a href="get_error_info.html">get_error_info</a></span>&lt;errno_code&gt;(e) )
 120         std::cerr &lt;&lt; "OS says: " &lt;&lt; strerror(*c) &lt;&lt; "\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 <h3>See Also:</h3>
 125 <div class="RenoPageList"><a href="boost-exception.html">Boost Exception<br/>
 126 </a><a href="exception_types_as_simple_semantic_tags.html">Exception Types as Simple Semantic Tags<br/>
 127 </a><a href="frequently_asked_questions.html">Frequently Asked Questions<br/>
 128 </a><a href="tutorial_enable_error_info.html">Integrating Boost Exception in Existing Exception Class Hierarchies<br/>
 129 </a></div>
 130 </div>
 131 <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
 132 <!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
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