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   9    <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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  13    <title>libstdc++-v3 HOWTO:  Chapter 19: Diagnostics</title>
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  26 <h1 class="centered"><a name="top">Chapter 19:  Diagnostics</a></h1>
  27
  28 <p>Chapter 19 deals with program diagnostics, such as exceptions
  29    and assertions.  You know, all the things we wish weren't even
  30    necessary at all.
  31 </p>
  32
  33
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  35 <hr />
  36 <h1>Contents</h1>
  37 <ul>
  38    <li><a href="#1">Adding data to exceptions</a></li>
  39    <li><a href="#2">Exception class hierarchy diagram</a></li>
  40    <li><a href="#3">Concept checkers -- <strong>new and improved!</strong></a></li>
  41 </ul>
  42
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  46
  47 <h2><a name="1">Adding data to exceptions</a></h2>
  48    <p>The standard exception classes carry with them a single string as
  49       data (usually describing what went wrong or where the 'throw' took
  50       place).  It's good to remember that you can add your own data to
  51       these exceptions when extending the hierarchy:
  52    </p>
  53    <pre>
  54    struct My_Exception : public std::runtime_error
  55    {
  56      public:
  57        My_Exception (const string&amp; whatarg)
  58            : std::runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
  59        int  errno_at_time_of_throw() const { return e; }
  60        DBID id_of_thing_that_threw() const { return id; }
  61      protected:
  62        int    e;
  63        DBID   id;     // some user-defined type
  64    };
  65    </pre>
  66    <p>Return <a href="#top">to top of page</a> or
  67       <a href="../faq/index.html">to the FAQ</a>.
  68    </p>
  69
  70 <hr />
  71 <h2><a name="2">Exception class hierarchy diagram</a></h2>
  72    <p>At one point we were going to make up a PDF of the exceptions
  73       hierarchy, akin to the one done for the I/O class hierarchy.
  74       Time was our enemy.  Since then we've moved to Doxygen, which has
  75       the useful property of not sucking.  Specifically, when the source
  76       code is changed, the diagrams are automatically brought up to date.
  77       For the old way, we had to update the diagrams separately.
  78    </p>
  79    <p>There are several links to the Doxygen-generated pages from
  80       <a href="../documentation.html">here</a>.
  81    </p>
  82    <p>Return <a href="#top">to top of page</a> or
  83       <a href="../faq/index.html">to the FAQ</a>.
  84    </p>
  85
  86 <hr />
  87 <h2><a name="3">Concept checkers -- <strong>new and improved!</strong></a></h2>
  88    <p>Better taste!  Less fat!  Literally!</p>
  89    <p>In 1999, SGI added <em>concept checkers</em> to their implementation
  90       of the STL:  code which checked the template parameters of
  91       instantiated pieces of the STL, in order to insure that the parameters
  92       being used met the requirements of the standard.  For example,
  93       the Standard requires that types passed as template parameters to
  94       <code>vector</code> be &quot;Assignable&quot; (which means what you think
  95       it means).  The checking was done during compilation, and none of
  96       the code was executed at runtime.
  97    </p>
  98    <p>Unfortunately, the size of the compiler files grew significantly
  99       as a result.  The checking code itself was cumbersome.  And bugs
 100       were found in it on more than one occasion.
 101    </p>
 102    <p>The primary author of the checking code, Jeremy Siek, had already
 103       started work on a replacement implementation.  The new code has been
 104       formally reviewed and accepted into
 105       <a href="http://www.boost.org/libs/concept_check/concept_check.htm">the
 106       Boost libraries</a>, and we are pleased to incorporate it into the
 107       GNU C++ library.
 108    </p>
 109    <p>The new version imposes a much smaller space overhead on the generated
 110       object file.  The checks are also cleaner and easier to read and
 111       understand.
 112    </p>
 113    <p>For GCC 3.0 and 3.1 they are off by default.  They can be enabled at
 114       configure time with
 115       <a href="../configopts.html"><code>--enable-concept-checks</code></a>.
 116       For 3.1 you can instead #define _GLIBCPP_CONCEPT_CHECKS to enable them
 117       on a per-translation-unit basis.
 118    </p>
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 120       <a href="../faq/index.html">to the FAQ</a>.
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