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

Ordinary C++ Programming

Yes there are often places where this makes sense, and you have control over where and how you use your using stamenets. If this is a program with a limited scope you do not need to worry about place usings in global scope.

For monolithic applications I would strongly recommend not placing usings in a very busy namespace -- as collisions will drive you mad once you are committed. For example:

1. ::
2. ::companyname
3. ::companyname::component
4. ::companyname::component::sub-component.

I would say that it is perfectly acceptable to start place using statements at level 3 or 4 -- thats where the chances of collisions start becoming very low.

Generic / TMP Programming

In generic programming and TMP using is often used to configure your libraries for a specific domain and this is often done through using statements.