(Please forgive me if this has already been discussed, I'm new to the list and wasn't able to find it.) I'm using Boost 1.34.1 under MSVC8 and am experiencing a compile warning that I thought was a bit odd. Whenever I include convenience.hpp, I and up with the following warning: d:\prgrmmng\libraries\boost_1_34_1\boost\filesystem\convenience.hpp(46) : warning C4245: 'argument' : conversion from 'int' to 'boost::filesystem::system_error_type', signed/unsigned mismatch I eventually tracked this down to the following code in path.hpp: # ifdef BOOST_WINDOWS_API typedef unsigned system_error_type; BOOST_FILESYSTEM_DECL errno_type lookup_errno( system_error_type sys_err_code ); # else typedef int system_error_type; inline errno_type lookup_errno( system_error_type sys_err_code ) { return sys_err_code; } # endif Why is system_error_type given a different type depending on the platform?