Pretty much C++ is C++; however, the APIs that are availablel to the Windows programmer are probably not available to the UNIX programmer. In UNIX/LINUX systems you have to use something like X-Windows or fltk for graphical programs.
POSIX generally has some sort of API available on most platforms. Cygwin and POSIX compatibility layer for Windows both provide POSIX to Windows although they require additional libraries and are probably a tad slower (if not the same) than WinAPI.
Although be very careful when using POSIX as it can contain some (very) naughty things.