std::basic_streambuf<CharT,Traits>::overflow

protected:
virtual int_type overflow( int_type ch = Traits::eof() );

The intent of this function is to transmit characters from the put area of the stream buffer to the associated character sequence.

Formally, this function ensures that there is space at the put area for at least one character. The base class version always fails, and a possibly-succeeding implementation can only be provided in derived classes (see implementation requirements). The standard library provides std::strstreambuf::overflow(), (until C++26)std::basic_stringbuf::overflow() and std::basic_filebuf::overflow().