std::codecvt::unshift, std::codecvt::do_unshift

public member function, calls the member function do_unshift of the most derived class.

if the encoding represented by this codecvt facet is state-dependent, and state represents a conversion state that is not the initial shift state, writes the characters necessary to return to the initial shift state. The characters are written to a character array whose first element is pointed to by to. No more than to_end-to characters are written. The parameter to_next is updated to point one past the last character written.

A value of type std::codecvt_base::result, indicating the success status as follows:

The non-converting specialization std::codecvt<char, char, std::mbstate_t> always returns std::codecvt_base::noconv

This function is called by std::basic_filebuf::close() and in other situations when finalizing a state-dependent multibyte character sequence.