basic_filebuf::overflow

virtual int_type overflow(int_type c = T::eof());

If c != T::eof(), the protected virtual member function endeavors to insert the element T::to_char_type(c) into the output buffer. It can do so in various ways:

• If a write position is available, it can store the element into the write position and increment the next pointer for the output buffer.
• It can make a write position available by allocating new or additional storage for the output buffer.
• It can convert any pending output in the output buffer, followed by c, by using the file conversion facet fac to call fac.out as needed. Each element x of type char thus produced is written to the associated stream designated by the file pointer fp as if by successive calls of the form fputc(x, fp). If any conversion or write fails, the function does not succeed.

If the function cannot succeed, it returns T::eof(). Otherwise, it returns T::not_eof(c).