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- <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
- <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
- <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
- <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 19." />
- <meta name="GENERATOR" content="vi and eight fingers" />
- <title>libstdc++-v3 HOWTO: Chapter 19</title>
-<link rel="StyleSheet" href="../lib3styles.css" />
-</head>
-<body>
-
-<h1 class="centered"><a name="top">Chapter 19: Diagnostics</a></h1>
-
-<p>Chapter 19 deals with program diagnostics, such as exceptions
- and assertions. You know, all the things we wish weren't even
- necessary at all.
-</p>
-
-
-<!-- ####################################################### -->
-<hr />
-<h1>Contents</h1>
-<ul>
- <li><a href="#1">Adding data to exceptions</a></li>
- <li><a href="#2">Exception class hierarchy diagram</a></li>
- <li><a href="#3">Concept checkers -- <strong>new and improved!</strong></a></li>
- <li><a href="#4">Verbose <code>terminate</code></a></li>
-</ul>
-
-<hr />
-
-<!-- ####################################################### -->
-
-<h2><a name="1">Adding data to exceptions</a></h2>
- <p>The standard exception classes carry with them a single string as
- data (usually describing what went wrong or where the 'throw' took
- place). It's good to remember that you can add your own data to
- these exceptions when extending the hierarchy:
- </p>
- <pre>
- struct My_Exception : public std::runtime_error
- {
- public:
- My_Exception (const string& whatarg)
- : std::runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
- int errno_at_time_of_throw() const { return e; }
- DBID id_of_thing_that_threw() const { return id; }
- protected:
- int e;
- DBID id; // some user-defined type
- };
- </pre>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="2">Exception class hierarchy diagram</a></h2>
- <p>At one point we were going to make up a PDF of the exceptions
- hierarchy, akin to the one done for the I/O class hierarchy.
- Time was our enemy. Since then we've moved to Doxygen, which has
- the useful property of not sucking. Specifically, when the source
- code is changed, the diagrams are automatically brought up to date.
- For the old way, we had to update the diagrams separately.
- </p>
- <p>There are several links to the Doxygen-generated pages from
- <a href="../documentation.html">here</a>.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="3">Concept checkers -- <strong>new and improved!</strong></a></h2>
- <p>Better taste! Less fat! Literally!</p>
- <p>In 1999, SGI added <em>concept checkers</em> to their implementation
- of the STL: code which checked the template parameters of
- instantiated pieces of the STL, in order to insure that the parameters
- being used met the requirements of the standard. For example,
- the Standard requires that types passed as template parameters to
- <code>vector</code> be "Assignable" (which means what you think
- it means). The checking was done during compilation, and none of
- the code was executed at runtime.
- </p>
- <p>Unfortunately, the size of the compiler files grew significantly
- as a result. The checking code itself was cumbersome. And bugs
- were found in it on more than one occasion.
- </p>
- <p>The primary author of the checking code, Jeremy Siek, had already
- started work on a replacement implementation. The new code has been
- formally reviewed and accepted into
- <a href="http://www.boost.org/libs/concept_check/concept_check.htm">the
- Boost libraries</a>, and we are pleased to incorporate it into the
- GNU C++ library.
- </p>
- <p>The new version imposes a much smaller space overhead on the generated
- object file. The checks are also cleaner and easier to read and
- understand.
- </p>
- <p>For GCC 3.0 and 3.1 they are off by default. They can be enabled at
- configure time with
- <a href="../configopts.html"><code>--enable-concept-checks</code></a>.
- For 3.1 you can instead #define _GLIBCPP_CONCEPT_CHECKS to enable them
- on a per-translation-unit basis.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="4">Verbose <code>terminate</code></a></h2>
- <p>If you are having difficulty with uncaught exceptions and want a
- little bit of help debugging the causes of the core dumps, you can
- make use of a GNU extension in GCC 3.1 and later:
- </p>
- <pre>
- #include <exception>
-
- int main()
- {
- std::set_terminate (__gnu_cxx::__verbose_terminate_handler);
- ...
- throw <em>anything</em>;
- }</pre>
- <p>The <code> __verbose_terminate_handler </code> function obtains the name
- of the current exception, attempts to demangle it, and prints it to
- stderr. If the exception is derived from <code> std::exception </code>
- then the output from <code>what()</code> will be included.
- </p>
- <p>Any replacement termination function is required to kill the program
- without returning; this one calls abort.
- </p>
- <p>For example:
- </p>
- <pre>
- #include <exception>
- #include <stdexcept>
-
- struct BLARGH : std::runtime_error
- {
- BLARGH (const string& whatarg)
- : std::runtime_error(whatarg) { }
- };
-
- int main (int argc)
- {
- std::set_terminate (__gnu_cxx::__verbose_terminate_handler);
- if (argc > 5)
- throw BLARGH("argc is greater than 5!");
- else
- throw argc;
- }</pre>
- <p>In GCC 3.1 and later, this gives
- </p>
- <pre>
- % ./a.out
- terminate called after throwing a `int'
- Aborted
- % ./a.out f f f f f f f f f f f
- terminate called after throwing a `BLARGH'
- what(): argc is greater than 5!
- Aborted
- %</pre>
- <p>The 'Aborted' line comes from the call to abort(), of course.
- </p>
- <p><strong>UPDATE:</strong> Starting with GCC 3.4, this is the default
- termination handler; nothing need be done to use it. To go back to
- the previous "silent death" method, simply include
- <code><exception><code> and <code><cstdlib><code>, and call
- </p>
- <pre>
- std::set_terminate (std::abort);</pre>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-
-<!-- ####################################################### -->
-
-<hr />
-<p class="fineprint"><em>
-See <a href="../17_intro/license.html">license.html</a> for copying conditions.
-Comments and suggestions are welcome, and may be sent to
-<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
-</em></p>
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