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  15     In 1999, SGI added <span class="quote">“<span class="quote">concept checkers</span>”</span> to their
  16     implementation of the STL: code which checked the template
  17     parameters of instantiated pieces of the STL, in order to insure
  18     that the parameters being used met the requirements of the
  19     standard.  For example, the Standard requires that types passed as
  20     template parameters to <code class="classname">vector</code> be
  21     "Assignable" (which means what you think it means).  The
  22     checking was done during compilation, and none of the code was
  23     executed at runtime.
  24    </p><p>
  25      Unfortunately, the size of the compiler files grew significantly
  26      as a result.  The checking code itself was cumbersome.  And bugs
  27      were found in it on more than one occasion.
  28    </p><p>
  29      The primary author of the checking code, Jeremy Siek, had already
  30      started work on a replacement implementation.  The new code was
  31      formally reviewed and accepted into
  32    <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
  33    Boost libraries</a>, and we are pleased to incorporate it into the
  34    GNU C++ library.
  35  </p><p>
  36    The new version imposes a much smaller space overhead on the generated
  37    object file.  The checks are also cleaner and easier to read and
  38    understand.
  39  </p><p>
  40    They are off by default for all versions of GCC.
  41    They can be enabled at configure time with
  42    <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
  43    You can enable them on a per-translation-unit basis with
  44      <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
  45  </p><p>
  46    Please note that the checks are based on the requirements in the original
  47    C++ standard, some of which have changed in the new C++11 revision.
  48    Additionally, some correct code might be rejected by the concept checks,
  49    for example template argument types may need to be complete when used in
  50    a template definition, rather than at the point of instantiation.
  51    There are no plans to address these shortcomings.
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