Method do_length

test/path_unit_test.cpp:1047–1047

    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; }
};