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| <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" /> |
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| <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> |
| |
| |
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| <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> |
| </ul> |
| |
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| |
| <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>They are off by default for all versions of GCC from 3.0 to 3.4 (the |
| latest release at the time of writing). |
| They can be enabled at configure time with |
| <a href="../configopts.html"><code>--enable-concept-checks</code></a>. |
| You can enable them on a per-translation-unit basis with |
| <code>#define _GLIBCXX_CONCEPT_CHECKS</code> for GCC 3.4 and higher |
| (or with <code>#define _GLIBCPP_CONCEPT_CHECKS</code> for versions |
| 3.1, 3.2 and 3.3). |
| </p> |
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| <a href="../faq/index.html">to the FAQ</a>. |
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