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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>
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<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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