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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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