[pjmlp]

Well it is just plain old COM, with a new base interface IIspectable that inherits from IUnknown and using .NET metadata instead of TLB files.

Using it from C++/CX is quite painless compared with the old ways, as it is quite similar to .NET, Delphi and C++ Builder ways of using COM.

For a little pain ATL style, you can use Windows Runtime Library.

Kenny Kerr and his team are pushing for C++/WinRT, a new C++ projection (language binding), based on pure Modern C++, with the goal to eventually replace C++/CX. Currently WIP, though.

For those that really like to suffer, the old way of using MIDL compiler with C is still possible for UWP applications.

From our projects, I would say most UWP developers use .NET for UWP controls and only resort to C++/CX if integration is C++ code is required.