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6    In 1999, SGI added <span class="quote">���<span class="quote">concept checkers</span>���</span> to their
7    implementation of the STL: code which checked the template
8    parameters of instantiated pieces of the STL, in order to insure
9    that the parameters being used met the requirements of the
10    standard.  For example, the Standard requires that types passed as
11    template parameters to <code class="classname">vector</code> be
12    "Assignable" (which means what you think it means).  The
13    checking was done during compilation, and none of the code was
14    executed at runtime.
15   </p><p>
16     Unfortunately, the size of the compiler files grew significantly
17     as a result.  The checking code itself was cumbersome.  And bugs
18     were found in it on more than one occasion.
19   </p><p>
20     The primary author of the checking code, Jeremy Siek, had already
21     started work on a replacement implementation.  The new code was
22     formally reviewed and accepted into
23   <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
24   Boost libraries</a>, and we are pleased to incorporate it into the
25   GNU C++ library.
26 </p><p>
27   The new version imposes a much smaller space overhead on the generated
28   object file.  The checks are also cleaner and easier to read and
29   understand.
30 </p><p>
31   They are off by default for all versions of GCC.
32   They can be enabled at configure time with
33   <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
34   You can enable them on a per-translation-unit basis with
35     <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
36 </p><p>
37   Please note that the checks are based on the requirements in the original
38   C++ standard, many of which were relaxed in the C++11 standard and so valid
39   C++11 code may be incorrectly rejected by the concept checks.  Additionally,
40   some correct C++03 code might be rejected by the concept checks,
41   for example template argument types may need to be complete when used in
42   a template definition, rather than at the point of instantiation.
43   There are no plans to address these shortcomings.
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