Hello,

if you would only use ISO standard C++ language/classes it would be possible but normally this is not the case.

for example the user interface (there are completly different API's under Windows/Linux for this) (but you can run a C++ program written for Windows user interface with "Wine" (a Windows Emulator for Linux))

there're almost unlimited api's that are either completly different on windows/linux or not available at all on one of these platform's.

it all depends what kinda program you want to write -> what features it needs -> what API's it has to deal with.

having said this I think it's very very unlikely that you can compile to a windows/linux version from the same source without modification

if you're lucky you'll get away with a few #ifdef statements in some header files ..
if you're unlucky you have to write some parts of the application multiple times for each operating systems you want to support

Regards
Christian