Right I know what a C++ ABI is but I still don't get it. To write C++ apps on Linux you need to match the ABI because there are lots of raw libraries that are just straight compiled C++. On Windows it's not like that outside of the VC++ runtime itself. Everything is C APIs or COM, and wrappers around that. MS never documented their C++ ABI because as far as they're concerned it's not something anyone needs to know, in order to write Windows apps or have components/libraries interop with each other. They never expose raw C++ objects in Windows. When they have C++ APIs they come as source or headers.