Win32 doesn't support UTF-8 filenames natively. That's why boost::filesystem would have to convert t o/from UCS-2 along Win32 interface boundaries. If you're concerned about other platforms, you shouldn't be. boost::filesystem currently works only with latin encodings in ascii strings so no functionality would be taken away.
Not true: currently the narrow character strings passed to boost.filefsystem are assumed to be in that platforms native encoding - you can for example pass native Windows narrow character strings (not just ACSII ones) to the lib, and actually I don't see why you can't pass UTF-8 on Linux. What you can't do is use the same encoding on all platforms, because the underlying platform API's won't understand them. Heres my (non-portable) test code BTW: namespace fs = boost::filesystem; int _tmain(int argc, _TCHAR* argv[]) { const char* name = "étrange.txt"; fs::path p(name, fs::native); fs::ofstream os(p); os << "Ha! Ha! Ha!"; os.close(); assert(fs::exists(p)); assert(!fs::is_directory(p)); assert(!fs::is_empty(p)); assert(fs::file_size(p)); assert(fs::remove(p)); return 0; } John.