Concept Checking
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## Concept Checking

In 1999, SGI added “concept checkers” 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 `vector` be "Assignable" (which means what you think it means). The checking was done during compilation, and none of the code was executed at runtime.

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.

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 the Boost libraries, and we are pleased to incorporate it into the GNU C++ library.

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.

They are off by default for all versions of GCC. They can be enabled at configure time with `--enable-concept-checks`. You can enable them on a per-translation-unit basis with `-D_GLIBCXX_CONCEPT_CHECKS`.

Please note that the checks are based on the requirements in the original C++ standard, many of which were relaxed in the C++11 standard and so valid C++11 code may be incorrectly rejected by the concept checks. Additionally, some correct C++03 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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