Why does boost::filesystem::path::string() return by value on Windows and by reference on POSIX?

Question

From boost/filesystem/path.hpp:

#   ifdef BOOST_WINDOWS_API
    const std::string string() const
    {
      [...]
    }
#   else   // BOOST_POSIX_API
    //  string_type is std::string, so there is no conversion
    const std::string&  string() const { return m_pathname; }
    [...]
#   endif

For wstring() it is exactly the other way around - returning by reference on Windows and by value on POSIX. Is there an interesting reason for that?

Answer 1

On Windows, path stores a wstring, since the only way to handle Unicode-encoded paths in Windows is with UTF-16. On other platforms, the filesystems handle Unicode via UTF-8 (or close enough), so on those platforms, path stores a string.

So on non-Windows platforms, path::string will return a const-reference to the actual internal data structure. On Windows, it has to generate a std::string, so it returns it by copy.

Note that the File System TS bound for C++17 does not do this. There, path::string will always return a copy. If you want the natively stored string type, you must use path::native, whose type will be platform-dependent.