std::basic_filebuf<CharT,Traits>::overflow
From cppreference.com
< cpp | io | basic filebuf
| protected: virtual int_type overflow( int_type ch = Traits::eof() ); |
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Writes some data from the put area to the associated character sequence (to the file).
Behaves like the base class version std::basic_streambuf::overflow(), except that the behavior of “consuming characters” is defined as follows:
1) First, uses std::codecvt::out of the imbued locale to convert the characters into external (possibly multibyte) representation, stored in a temporary buffer, as follows: (XSIZE is some unspecified buffer size)
const std::codecvt<CharT, char, typename Traits::state_type>& a_codecvt = std::use_facet<