istream
Class istream objects are specialized for input; as for
ostream, they are derived from ios, so you can use any of
the general-purpose methods from that base class. Declarations for this
class also come from `iostream.h'.
Constructor: istream::istream ()
When used without arguments, the istream constructor simply
allocates a new ios object and initializes the input counter (the
value reported by istream::gcount) to 0.
Constructor: istream::istream (streambuf *sb [, ostream tie])
You can also call the constructor with one or two arguments. The first
argument sb is a streambuf*; if you supply this pointer,
the constructor uses that streambuf for input.
You can use the second optional argument tie to specify a related
output stream as the initial value for ios::tie.
If you give the istream a streambuf explicitly, using
this constructor, the sb is not destroyed (or deleted or
closed) when the ostream is destroyed.