basic_streambuf::underflow

virtual int_type underflow();

The protected virtual member function endeavors to extract the current element c from the input stream, without advancing the current stream position, and return it as T::to_int_type(c). It can do so in various ways:

• If a read position is available, c is the element stored in the read position.
• It can make a read position available by allocating new or additional storage for the input buffer, then reading 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 in the input stream, converted as described above. The default behavior is to return T::eof().