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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 29. Compile Time Checks</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="extensions.html" title="Part XII. Extensions" /><link rel="prev" href="bk01pt12pr03.html" title="" /><link rel="next" href="debug_mode.html" title="Chapter 30. Debug Mode" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 29. Compile Time Checks</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt12pr03.html">Prev</a> </td><th width="60%" align="center">Part XII. Extensions</th><td width="20%" align="right"> <a accesskey="n" href="debug_mode.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.compile_checks"></a>Chapter 29. Compile Time Checks</h2></div></div></div><p>
Also known as concept checking.
</p><p>In 1999, SGI added <span class="emphasis"><em>concept checkers</em></span> 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 class="code">vector</code> be “<span class="quote">Assignable</span>” (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 class="ulink" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">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 class="ulink" href="../configopts.html" target="_top"><code class="literal">--enable-concept-checks</code></a>.
You can enable them on a per-translation-unit basis with
<code class="code">#define _GLIBCXX_CONCEPT_CHECKS</code> for GCC 3.4 and higher
(or with <code class="code">#define _GLIBCPP_CONCEPT_CHECKS</code> for versions
3.1, 3.2 and 3.3).
</p><p>Please note that the upcoming C++ standard has first-class
support for template parameter constraints based on concepts in the core
language. This will obviate the need for the library-simulated concept
checking described above.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt12pr03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extensions.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug_mode.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"> </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 30. Debug Mode</td></tr></table></div></body></html>
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