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15 <a href=
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17 <h1>Boost Exception
</h1>
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>
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>
37 "Unexpected exception, diagnostic information follows:\n" <<
38 <span class=
"RenoLink"><a href=
"current_exception_diagnostic_information.html">current_exception_diagnostic_information
</a></span>();
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
<struct _iobuf
> __cdecl my_fopen(const char *,const char *)
42 Dynamic exception type: class boost::exception_detail::clone_impl
<class fopen_error
>
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
&) 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> & e )
57 e
<span class=
"RenoLink"><a href=
"exception_operator_shl.html"><<</a></span> foo_info(foo);
58 throw e; //Compile error: boost::
<span class=
"RenoLink"><a href=
"exception.html">exception
</a></span> is abstract
60 <p>The correct code is:
</p>
61 <pre>catch( boost::
<span class=
"RenoLink"><a href=
"exception.html">exception
</a></span> & e )
63 e
<span class=
"RenoLink"><a href=
"exception_operator_shl.html"><<</a></span> foo_info(foo);
64 throw; //Okay, re-throwing the original exception object.
66 <h3>Why use operator
<< 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
<<</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"><<</a></span> foo_info(foo)
<span class=
"RenoLink"><a href=
"exception_operator_shl.html"><<</a></span> bar_info(bar);
</pre>
69 <h3>Why is operator
<< 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"><<</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
<<</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> |
<a href=
"motivation.html">Motivation
</a></span>
79 <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. -->
80 <!-- Distributed under the Boost Software License, Version 1.0. (See accompanying -->
81 <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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86 <small>Copyright (c)
2006-
2009 by Emil Dotchevski and Reverge Studios, Inc.
<br/>
87 Distributed under the
<a href=
"http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version
1.0</a>.
</small>