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──The Microsoft iostream Classes────────────────────────────────────────────
 
  virtual int underflow() = 0;
 
  Remarks
 
  The virtual underflow function, together with the sync and overflow
  functions, defines the characteristics of the streambuf-derived class.
  Each derived class might implement underflow differently, but the
  interface with the calling stream class is the same.
 
  The underflow function is most frequently called by public streambuf
  functions like sgetc and sgetn when they sense that the get area is
  empty, but other classes, including the stream classes, can call
  underflow anytime.
 
  The underflow function supplies the get area with characters from the
  input source. If the get area contains characters, then underflow
  returns the first character. If the get area is empty, then it fills the
  get area and returns the next character (which it leaves in the get
  area). If there are no more characters available, then underflow returns
  EOF and leaves the get area empty.
 
  In the strstreambuf class, underflow adjusts the egptr pointer to access
  storage that was dynamically allocated by a call to overflow.
 
  Default Implementation
 
  No default implementation. Derived classes must define this function.
 
 
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