Standard C++ will look the same everywhere. What differs is if you use different libraries, e.g. the WinAPI on Windows. You can't use the WinAPI on Linux so you will have to use something else. Some compilers have language extensions e.g. GCC's variable length arrays. If you want be cross platform you should try to stay away from language extensions and use cross platform libraries.
Anything from #include <windows.h> won't work on Linux. Anything from <sys/...> won't work on Windows. If you stay away from those headers you'll generally be ok. Some 3rd party libraries (aka DirectX) will be platform specific as well, but anything in std:: is safe to use.