Chapter 16. Compile Time Checks
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## Chapter 16. Compile Time Checks

Also known as 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 has been 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 from 3.0 to 3.4 (the latest release at the time of writing). They can be enabled at configure time with `--enable-concept-checks`. You can enable them on a per-translation-unit basis with `#define _GLIBCXX_CONCEPT_CHECKS` for GCC 3.4 and higher (or with `#define _GLIBCPP_CONCEPT_CHECKS` for versions 3.1, 3.2 and 3.3).

Please note that the concept checks only validate the requirements of the old C++03 standard. C++11 was expected to have first-class support for template parameter constraints based on concepts in the core language. This would have obviated the need for the library-simulated concept checking described above, but was not part of C++11.

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