I have changed my stance on “using” in C++ headers

I used to be pretty strictly against using either C++ using-directives or -declarations from within header files. It kind of stuck with me as a no-go. But that has changed in recent years.

There are now good cases where using can go into a header. For example, I do not really like putting things like…

using namespace std::string_literals;
using namespace std::string_view_literals;
using namespace std::chrono_literals;

…at the beginning of each source file. Did you know that you can pull all those (and some more) in with a single using namespace std::literals? Either way, in my newer projects, these usually go into one of the more prominent headers. Same goes for other literal operators such as those from the SI library. And so do using declarations for common vocabulary types. E.g. 2D or 3D vector types , in math heavy projects. Of course, they always go after the specific #include(s) the using is referencing. The benefits of doing that usually outweigh the danger of name-clashes and weird order dependencies.

There are cases where I still avoid using in headers however, and that is when the given header is ‘public’, i.e. being consumed by something that is not under my organization’s control. In that case, you better leave that decision to the library consumer.