basic_filebuf::underflow

virtual int_type underflow();

The protected virtual member function endeavors to extract the current element c from the input stream, and return the element as T::to_int_type(c). It can do so in various ways:

• If a read position is available, it takes c as the element stored in the read position and advances the next pointer for the input buffer.
• It can read one or more elements of type char, as if by successive calls of the form fgetc(fp), and convert them to an element c of type E by using the file conversion facet fac to call fac.in as needed. If any read or conversion fails, the function does not succeed.

If the function cannot succeed, it returns T::eof(). Otherwise, it returns c, converted as described above.