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  15     Also known as concept checking.
  16   </p><p>In 1999, SGI added <span class="emphasis"><em>concept checkers</em></span> to their implementation
  17       of the STL:  code which checked the template parameters of
  18       instantiated pieces of the STL, in order to insure that the parameters
  19       being used met the requirements of the standard.  For example,
  20       the Standard requires that types passed as template parameters to
  21       <code class="code">vector</code> be <span class="quote">“<span class="quote">Assignable</span>”</span> (which means what you think
  22       it means).  The checking was done during compilation, and none of
  23       the code was executed at runtime.
  24    </p><p>Unfortunately, the size of the compiler files grew significantly
  25       as a result.  The checking code itself was cumbersome.  And bugs
  26       were found in it on more than one occasion.
  27    </p><p>The primary author of the checking code, Jeremy Siek, had already
  28       started work on a replacement implementation.  The new code has been
  29       formally reviewed and accepted into
  30       <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
  31       Boost libraries</a>, and we are pleased to incorporate it into the
  32       GNU C++ library.
  33    </p><p>The new version imposes a much smaller space overhead on the generated
  34       object file.  The checks are also cleaner and easier to read and
  35       understand.
  36    </p><p>They are off by default for all versions of GCC from 3.0 to 3.4 (the
  37       latest release at the time of writing).
  38       They can be enabled at configure time with
  39       <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
  40       You can enable them on a per-translation-unit basis with
  41       <code class="code">#define _GLIBCXX_CONCEPT_CHECKS</code> for GCC 3.4 and higher
  42       (or with <code class="code">#define _GLIBCPP_CONCEPT_CHECKS</code> for versions
  43       3.1, 3.2 and 3.3).
  44    </p><p>Please note that the upcoming C++ standard has first-class
  45    support for template parameter constraints based on concepts in the core
  46    language. This will obviate the need for the library-simulated concept
  47    checking described above.
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