namespaces - Is it wrong to use C++ 'using' keyword in a header file?

Question

I've been told it's bad to have "using namespace ns123" in a header file, but I can't remember what the reason given was. Is it in fact a bad thing to do, and why?

Answer 1

It's a bad practice, in general, because it defeats the purpose of namespaces. By defining in a header you're not enforcing strict control over the scope of the using declaration, meaning that you can run into name clashes in unexpected places.