Chapter 16. Compile Time Checks

Chapter 16. Compile Time Checks
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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 upcoming C++ standard has first-class + support for template parameter constraints based on concepts in the core + language. This will obviate the need for the library-simulated concept + checking described above. +

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