“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

You are right that using namespace in header is dangerous. I do not know a way how to undo it. It is easy to detect it however just search for using namespace in header files. For that last reason it is uncommon in real projects. More experienced coworkers will soon complain if someone does something like it.

In real projects people try to minimize the amount of included files, because the less you include the quicker it compiles. That saves time of everybody. However if the header file assumes that something should be included before it then it should include it itself. Otherwise it makes headers not self-contained.