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, 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.