I've finished a library of functions for calculating sunblock data, in C++. Is it possible to use my C++ library for iOS or Android apps?
Yes, although since running apps from the command line is not for everyone, you should also use some kind of GUI or TUI with it.
WinAPI (Windows), uses <windows.h>
Some of those may not be what you want, but at least one or two of them should work for you!