Unknown type name 'using' in Xcode

Question

I have a project (MyLib) inside my main project (MainApp). When I create MainApp, Xcode gives me an error in one of the .h MyLib files:

using namespace cv; 

Error message:

 Unknown type name 'using' 

If I create only MyLib, I have no errors.

Answer 1

I expect the header file to be included from a .m file (Objective-C)?

Change the extension to .mm (Objective-C ++) and it should work fine.