Method do_length

test/path_unit_test.cpp:880–881

    virtual std::codecvt_base::result do_unshift(std::mbstate_t&,
        char*, char*, char* &) const  { return ok; }
    virtual int do_length(std::mbstate_t &,
      const char*, const char*, std::size_t) const  { return 0; }
    virtual int do_max_length() const throw () { return 0; }
  };