“using namespace” in c++ headers

Question

In all our c++ courses, all the teachers always put using namespace std; right after the #includes in their .h files. This seems to me

Answer 1

With regards to "Is there some way to undo [a using declaration]?"

I think it is useful to point out that using declarations are affected by scope.

#include 

{   // begin a new scope with {
    using namespace std;
    vector myVector;  // std::vector is used
}   // end the scope with }

vector myOtherVector;   // error vector undefined
std::vector mySTDVector // no error std::vector is fully qualified

So effectively yes. By limiting the scope of the using declaration its effect only lasts within that scope; it is 'undone' when that scope ends.

When the using declaration is declared in a file outside of any other scope it has file-scope and affects everything in that file.

In the case of a header file, if the using declaration is at file-scope this will extend to the scope of any file the header is included in.