>1 namespaces?

[johnhoffman]

What does it mean to use two namespaces at the same time? For instance, if I use

  using namespace cv;
  using namespace std;

how does the compiler resolve conflicts between function names in these namespaces?

[Cubbi]

It doesn't; if you use an unqualified function name and there are matching functions in both namespaces, compilation will stop.

[johnhoffman]

Thanks? What's an unqualified function name? One that appears in both namespaces?

[Cubbi]

unqualified is one that doesn't have a :: before it. endl() is unqualified, so it will be looked up in all namespaces you pulled in with using declarations.
std::endl() and ::std::endl() are qualified, so they will only be looked up in the std namespace