Should I include stddef.h or cstddef for size_t

Question

When I want to use size_t in C++, should I include or ? I have heard several people saying that

Answer 1

Both are in the standard and, AFAIK, there to stay.

The form cXXX always introduces the names in the std namespaces, the form XXX.h always introduces the names in the global namespace. Both may also put the names in the other namespace (at least in C++0X, it wasn't the case previously. As respecting that constraint make it impossible to build a C++ library from a C library you don't control, the constraint was removed. g++ suffers of that problem at least on the non glibc targets).

For traditional Unix headers, in all implementation I've tested the form XXX.h includes the additional Unix identifier if you have the needed feature macros defined before. The behavior for the form cXXX was inconsistent between implementations. So in practice I use the XXX.h as I often need those declarations.