Editing C++ (section)

===Don't put 'using namespace' in a header===

For the same reason that you generally don't want to define macros in a header file.  You don't want to force it on anyone who is using your header.  There is presumably a reason why the objects were put into a namespace in the first place; using 'using namespace' puts the whole namespace into global namespace.  It's fine however to use 'using namespace' in your .cxx file if you find that convenient, because it affects only your .cxx file.

Possible exception: 'using namespace std' is often seen in header files.  I still don't think it's a good idea though.