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