In visualc.hpp I found the following:
#if !defined(_MSC_EXTENSIONS) && !defined(BOOST_DISABLE_WIN32) # define BOOST_DISABLE_WIN32 #endif
I.e., disabling MSVC-specific exnetions (/Za) disables also WIN32, which, in turn, leads to undefined BOOST_WINDOWS. So if I try to compile my project with /Za, nothing compiles, because there're lot of places where BOOST_WINDOWS is used to select platform-dependant implementation (eg., in ASIO). But if I understand correctly, the purpose of disabling MSVC *language* extenstions is to ensure that the code is portable & standard-complient. So it seems to be incorrect to imply from this BOOST_DISABLE_WIN32.
The point is that with /Za you can't #include windows.h or use any platform specific features, so that makes it very hard (impossible?) to compile code that is binary compatible between /Za and not /Za. I realize there are exceptions to this - particularly for libraries where the implementation is confined to an external binary - but given that most of Boost is header only, what did you expect? Regards, John