Usually you can mix it too, while U++ containers have different element requirements (either more constrained in Vector or totally relaxed in Array), there is huge intersection where std:: algorithms work on U++ containers, std:: containers work with U++ concrete types and U++ algorithms work on std:: containers.
Thanks ! I will definitely take a look at U++ for cross-platform C++ programming.
PS.: The only thing prohibited is using std:: concretes in U++ Vector-flavor containers. E.g. Vector<std::string> is invalid, because std::string is not guaranteed to satisfy "moveability" constraint. This might cause some smaller issues, as you need to convert str::string to Upp::String before putting e.g. into U++ widgets.
OTOH, Upp::String has implicit conversion to/from std::string...