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4 In 1999, SGI added “<span class="quote">concept checkers</span>” to their
5 implementation of the STL: code which checked the template
6 parameters of instantiated pieces of the STL, in order to insure
7 that the parameters being used met the requirements of the
8 standard. For example, the Standard requires that types passed as
9 template parameters to <code class="classname">vector</code> be
10 "Assignable" (which means what you think it means). The
11 checking was done during compilation, and none of the code was
14 Unfortunately, the size of the compiler files grew significantly
15 as a result. The checking code itself was cumbersome. And bugs
16 were found in it on more than one occasion.
18 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="ulink" 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
25 The new version imposes a much smaller space overhead on the generated
26 object file. The checks are also cleaner and easier to read and
29 They are off by default for all versions of GCC.
30 They can be enabled at configure time with
31 <a class="ulink" href="../configopts.html" target="_top"><code class="literal">--enable-concept-checks</code></a>.
32 You can enable them on a per-translation-unit basis with
33 <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
35 Please note that the upcoming C++ standard has first-class
36 support for template parameter constraints based on concepts in the core
37 language. This will obviate the need for the library-simulated concept
38 checking described above.
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