Partly this depends on the platform. C++ is well supported on Microsoft's .NET platform where you can access all the functionality of the .NET libraries through C++.

STL, I guess, is more used on Linux. I would advise against trying to use portable libraries and instead using libraries designed for the platform you are targeting.

Having said that, a good portable UI library is the open source WxWidgets which is accessible through C++ for OSX, Linux, Windows