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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
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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>They are off by default for all versions of GCC from 3.0 to 3.4 (the
114      latest release at the time of writing).
115      They can be enabled at configure time with
116      <a href="../configopts.html"><code>--enable-concept-checks</code></a>.
117      You can enable them on a per-translation-unit basis with
118      <code>#define _GLIBCXX_CONCEPT_CHECKS</code> for GCC 3.4 and higher
119      (or with <code>#define _GLIBCPP_CONCEPT_CHECKS</code> for versions
120      3.1, 3.2 and 3.3).
121   </p>
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