Can I undo the effect of "using namespace" in C++?

Question

With using namespace I make the whole contents of that namespace directly visible without using the namespace qualifier. This can cause problems if using namespace occurs in widely used headers - we can unintendedly make two namespaces with identical classes names visible and the compiler will refuse to compile unless the class name is prepended with the namespace qualifier.

Can I undo using namespace so that the compiler forgets that it saw it previously?

Answer 1

No, but you can tell your coworkers that you should never have a using directive or declaration in a header.

Answer 2

As others said, you can't and the problem shouldn't be there in the first place.
The next-best thing you can do is bring in your needed symbols so that they are preferred by the name look-up:

namespace A { class C {}; } namespace B { class C {}; } using namespace A; using namespace B;  namespace D {     using A::C; // fixes ambiguity     C c; } 

In some cases you can also wrap the offending includes with a namespace:

namespace offender { #  include "offender.h" }