basic_streambuf::uflow

virtual int_type uflow();

The protected virtual member function endeavors to extract the current element c from the input stream, then advance the current stream position, 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 an element directly, from some external source, and deliver it as the value c.
• For a stream buffer with common input and output streams, it can make a read position available by writing out, to some external destination, some or all of the elements between the beginning and next pointers for the output buffer. Or it can allocate new or additional storage for the input buffer. The function then reads in, from some external source, one or more elements.

If the function cannot succeed, it returns T::eof(), or throws an exception. Otherwise, it returns the current element c in the input stream, converted as described above, and advances the next pointer for the input buffer. The default behavior is to call underflow() and, if that function returns T::eof(), to return T::eof(). Otherwise, the function returns the current element c in the input stream, converted as described above, and advances the next pointer for the input buffer.