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   7     In 1999, SGI added <span class="quote">“<span class="quote">concept checkers</span>”</span> to their
   8     implementation of the STL: code which checked the template
   9     parameters of instantiated pieces of the STL, in order to insure
  10     that the parameters being used met the requirements of the
  11     standard.  For example, the Standard requires that types passed as
  12     template parameters to <code class="classname">vector</code> be
  13     "Assignable" (which means what you think it means).  The
  14     checking was done during compilation, and none of the code was
  15     executed at runtime.
  16    </p><p>
  17      Unfortunately, the size of the compiler files grew significantly
  18      as a result.  The checking code itself was cumbersome.  And bugs
  19      were found in it on more than one occasion.
  20    </p><p>
  21      The primary author of the checking code, Jeremy Siek, had already
  22      started work on a replacement implementation.  The new code has been
  23      formally reviewed and accepted into
  24    <a class="ulink" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
  25    Boost libraries</a>, and we are pleased to incorporate it into the
  26    GNU C++ library.
  27  </p><p>
  28    The new version imposes a much smaller space overhead on the generated
  29    object file.  The checks are also cleaner and easier to read and
  30    understand.
  31  </p><p>
  32    They are off by default for all versions of GCC.
  33    They can be enabled at configure time with
  34    <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
  35    You can enable them on a per-translation-unit basis with
  36      <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
  37  </p><p>
  38    Please note that the upcoming C++ standard has first-class
  39    support for template parameter constraints based on concepts in the core
  40    language. This will obviate the need for the library-simulated concept
  41    checking described above.
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