Side note: The Win32 API has not been removed and older applications using the traditional application execution environment will continue to work as expected.
User interfaces in C++ will be written primarily in XAML. This libraries for working with XAML have all been ported to C++ and are compiled to native x86. Metro applications written with XAML and C++ do not run on top of .NET, they get compiled directly to x86 just like any other Visual C++ application.
Calling methods on UI controls is just like calling methods on any other object in C++. At the machine code level one pushes the this pointer onto the stack and then invokes a function via a v-table. This allows for the best possible performance even on low power devices.
Libraries used by modern C++ applications such as Boost are supported.