On cygwin, the following program

auto p = boost::filesystem::path{"foo"}; p /= "bar"; std::cout << p.string() << '\n';

outputs: foo\bar

but cygwin is a posix system and I expect: foo/bar

I read in the doc that "User-defined BOOST_POSIX_API and BOOST_WINDOWS_API macros are no longer supported."

I guess system/api_config.hpp should be: -# if defined(_WIN32) || defined(__CYGWIN__) // Windows default, including MinGW and Cygwin +# if defined(_WIN32) // Windows default, including MinGW # define BOOST_WINDOWS_API # else # define BOOST_POSIX_API # endif Frédéric