With the exception of Qt, none of those platforms are, in any way, shape or form, cross-platform. Visual C++ is Windows-only, and C++ Builder supports only Wind

With the exception of Qt, none of those platforms are, in any way, shape or form, cross-platform. Visual C++ is Windows-only, and C++ Builder supports only Windows and iOS (??). So you don't solve the problem, merely invert it. Instead of everything-except-Windows, you now support only-Windows. I also don't see how using VC++ for example circumvents API issues, since it's just a compiler. I can imagine that Qt might solve some issues, since it offers a very wide set of abstractions. As such, it can probably do things like convert string formats on-the-fly between getting it from the OS and passing it to your application.