ceph/src/boost/libs/exception/doc/frequently_asked_questions.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"><h2>Frequently Asked Questions</h2>
  23 </div>
  24 <h3>What is the cost of calling boost::throw_exception?</h3>
  25 <p>The cost is that boost::<span class="RenoLink"><a href="exception.html">exception</a></span> is added as a base of the exception emitted by boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span> (unless the passed type already derives from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>.)</p>
  26 <p>Calling boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span> does not cause dynamic memory allocations.</p>
  27 <h3>What is the cost of BOOST_THROW_EXCEPTION?</h3>
  28 <p>In addition to calling boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span>, <span class="RenoLink"><a href="BOOST_THROW_EXCEPTION.html">BOOST_THROW_EXCEPTION</a></span> invokes __FILE__, __LINE__ and the <span class="RenoLink"><a href="configuration_macros.html">BOOST_THROW_EXCEPTION_CURRENT_FUNCTION</a></span> macros. The space required to store the information is already included in sizeof(boost::<span class="RenoLink"><a href="exception.html">exception</a></span>).</p>
  29 <p>Calling <span class="RenoLink"><a href="BOOST_THROW_EXCEPTION.html">BOOST_THROW_EXCEPTION</a></span> does not cause dynamic memory allocations.</p>
  30 <h3>Should I use boost::throw_exception or BOOST_THROW_EXCEPTION or just throw?</h3>
  31 <p>The benefit of calling boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span> instead of using throw directly is that it ensures that the emitted exception derives from boost::<span class="RenoLink"><a href="exception.html">exception</a></span> and that it is compatible with boost::<span class="RenoLink"><a href="current_exception.html">current_exception</a></span>.</p>
  32 <p>The <span class="RenoLink"><a href="BOOST_THROW_EXCEPTION.html">BOOST_THROW_EXCEPTION</a></span> macro also results in a call to boost::<span class="RenoLink"><a href="throw_exception.html">throw_exception</a></span>, but in addition it records in the exception object the __FILE__ and __LINE__ of the throw, as well as the pretty name of the function that throws. This enables boost::<span class="RenoLink"><a href="diagnostic_information.html">diagnostic_information</a></span> to compose a more useful, if not user-friendly message.</p>
  33 <p>Typical use of boost::<span class="RenoLink"><a href="diagnostic_information.html">diagnostic_information</a></span> is:</p>
  34 <pre>catch(...)
  35     {
  36     std::cerr &lt;&lt;
  37         "Unexpected exception, diagnostic information follows:\n" &lt;&lt;
  38         <span class="RenoLink"><a href="current_exception_diagnostic_information.html">current_exception_diagnostic_information</a></span>();
  39     }</pre>
  40 <p>This is a possible message it may display -- the information in the first line is only available if <span class="RenoLink"><a href="BOOST_THROW_EXCEPTION.html">BOOST_THROW_EXCEPTION</a></span> was used to throw:</p>
  41 <pre>example_io.cpp(70): Throw in function class boost::shared_ptr&lt;struct _iobuf&gt; __cdecl my_fopen(const char *,const char *)
  42 Dynamic exception type: class boost::exception_detail::clone_impl&lt;class fopen_error&gt;
  43 std::exception::what: example_io error
  44 [struct boost::<span class="RenoLink"><a href="errinfo_api_function.html">errinfo_api_function</a></span>_ *] = fopen
  45 [struct boost::<span class="RenoLink"><a href="errinfo_errno.html">errinfo_errno</a></span>_ *] = 2, "No such file or directory"
  46 [struct boost::<span class="RenoLink"><a href="errinfo_file_name.html">errinfo_file_name</a></span>_ *] = tmp1.txt
  47 [struct boost::<span class="RenoLink"><a href="errinfo_file_open_mode.html">errinfo_file_open_mode</a></span>_ *] = rb</pre>
  48 <p>In some development environments, the first line in that message can be clicked to show the location of the throw in the debugger, so it's easy to set a break point and run again to see the unexpected throw in the context of its call stack.</p>
  49 <h3>Why doesn't boost::exception derive from std::exception?</h3>
  50 <p>Despite that <span class="RenoLink"><a href="using_virtual_inheritance_in_exception_types.html">virtual inheritance should be used in deriving from base exception types</a></span>, quite often exception types (including the ones defined in the standard library) don't derive from std::exception virtually.</p>
  51 <p>If boost::<span class="RenoLink"><a href="exception.html">exception</a></span> derives from std::exception, using the <span class="RenoLink"><a href="enable_error_info.html">enable_error_info</a></span> function with such user-defined types would introduce dangerous ambiguity which would break all catch(std::exception &amp;) statements.</p>
  52 <p>Of course, boost::<span class="RenoLink"><a href="exception.html">exception</a></span> should not be used to replace std::exception as a base type in exception type hierarchies. Instead, it should be included as a virtual base, in addition to std::exception (which should probably also be derived virtually.)</p>
  53 <h3>Why is boost::exception abstract?</h3>
  54 <p>To prevent exception-neutral contexts from erroneously erasing the type of the original exception when adding <span class="RenoLink"><a href="error_info.html">error_info</a></span> to an active exception object:</p>
  55 <pre>catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
  56     {
  57     e <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> foo_info(foo);
  58     throw e; //Compile error: boost::<span class="RenoLink"><a href="exception.html">exception</a></span> is abstract
  59     }</pre>
  60 <p>The correct code is:</p>
  61 <pre>catch( boost::<span class="RenoLink"><a href="exception.html">exception</a></span> &amp; e )
  62     {
  63     e <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> foo_info(foo);
  64     throw; //Okay, re-throwing the original exception object.
  65     }</pre>
  66 <h3>Why use operator&lt;&lt; overload for adding info to exceptions?</h3>
  67 <p>Before throwing an object of type that derives from boost::<span class="RenoLink"><a href="exception.html">exception</a></span>, it is often desirable to add one or more <span class="RenoLink"><a href="error_info.html">error_info</a></span> objects in it. The syntactic sugar provided by <span class="RenoLink"><a href="exception_operator_shl.html">operator&lt;&lt;</a></span> allows this to be done directly in a throw expression:</p>
  68 <pre>throw error() <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> foo_info(foo) <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> bar_info(bar);</pre>
  69 <h3>Why is operator&lt;&lt; allowed to throw?</h3>
  70 <p>This question is referring to the following issue. Consider this throw statement example:</p>
  71 <pre>throw file_open_error() <span class="RenoLink"><a href="exception_operator_shl.html">&lt;&lt;</a></span> file_name(fn);</pre>
  72 <p>The intention here is to throw a file_open_error, however if <span class="RenoLink"><a href="exception_operator_shl.html">operator&lt;&lt;</a></span> fails to copy the std::string contained in the file_name <span class="RenoLink"><a href="error_info.html">error_info</a></span> wrapper, a std::bad_alloc could propagate instead. This behavior seems undesirable to some programmers.</p>
  73 <p>Bjarne Stroustrup, The C++ Programming Language, 3rd Edition, page 371:</p>
  74 <blockquote><p><i>"Throwing an exception requires an object to throw.  A C++ implementation is required to have enough spare memory to be able to throw bad_alloc in case of memory exhaustion.  However, it is possible that throwing some other exception will cause memory exhaustion."</i></p></blockquote>
  75 <p>Therefore, the language itself does not guarantee that an attempt to throw an exception is guaranteed to throw an object of the specified type; propagating a std::bad_alloc seems to be a possibility even outside of the scope of Boost Exception.</p>
  76 </div><div class="RenoAutoDIV"><div class="RenoHR"><hr/></div>
  77 See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a>&nbsp;| <a href="motivation.html">Motivation</a></span>
  78 </div>
  79 <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
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