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6    Also known as concept checking.
7  </p><p>In 1999, SGI added <span class="emphasis"><em>concept checkers</em></span> to their implementation
8      of the STL:  code which checked the template parameters of
9      instantiated pieces of the STL, in order to insure that the parameters
10      being used met the requirements of the standard.  For example,
11      the Standard requires that types passed as template parameters to
12      <code class="code">vector</code> be <span class="quote">���<span class="quote">Assignable</span>���</span> (which means what you think
13      it means).  The checking was done during compilation, and none of
14      the code was executed at runtime.
15   </p><p>Unfortunately, the size of the compiler files grew significantly
16      as a result.  The checking code itself was cumbersome.  And bugs
17      were found in it on more than one occasion.
18   </p><p>The primary author of the checking code, Jeremy Siek, had already
19      started work on a replacement implementation.  The new code has been
20      formally reviewed and accepted into
21      <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
22      Boost libraries</a>, and we are pleased to incorporate it into the
23      GNU C++ library.
24   </p><p>The new version imposes a much smaller space overhead on the generated
25      object file.  The checks are also cleaner and easier to read and
26      understand.
27   </p><p>They are off by default for all versions of GCC from 3.0 to 3.4 (the
28      latest release at the time of writing).
29      They can be enabled at configure time with
30      <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
31      You can enable them on a per-translation-unit basis with
32      <a class="link" href="using_macros.html" title="Macros"><code class="code">#define
33      _GLIBCXX_CONCEPT_CHECKS</code></a> for GCC 3.4 and higher
34      (or with <code class="code">#define _GLIBCPP_CONCEPT_CHECKS</code> for versions
35      3.1, 3.2 and 3.3).
36   </p><p>Please note that the concept checks only validate the requirements
37   of the old C++03 standard. C++11 was expected to have first-class
38   support for template parameter constraints based on concepts in the core
39   language. This would have obviated the need for the library-simulated concept
40   checking described above, but was not part of C++11.
41   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ext_preface.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="../index.html">Home</a></td><td width="40%" align="right" valign="top">��Chapter��17.��Debug Mode</td></tr></table></div></body></html>