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(iostream-295.info)Operators

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Operators and Default Streams
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   The GNU iostream library, `libio', implements the standard input and
output facilities for C++.  These facilities are roughly analogous (in
their purpose and ubiquity, at least) with those defined by the C
`stdio' functions.

   Although these definitions come from a library, rather than being
part of the "core language", they are sufficiently central to be
specified in the latest working papers for C++.

   You can use two operators defined in this library for basic input and
output operations.  They are familiar from any C++ introductory
textbook: `<<' for output, and `>>' for input.  (Think of data flowing
in the direction of the "arrows".)

   These operators are often used in conjunction with three streams that
are open by default:

 - Variable: ostream cout
     The standard output stream, analogous to the C `stdout'.

 - Variable: istream cin
     The standard input stream, analogous to the C `stdin'.

 - Variable: ostream cerr
     An alternative output stream for errors, analogous to the C
     `stderr'.

For example, this bare-bones C++ version of the traditional "hello"
program uses `<<' and `cout':

     #include <iostream.h>
     
     int main(int argc, char **argv)
     {
       cout << "Well, hi there.\n";
       return 0;
     }

   Casual use of these operators may be seductive, but--other than in
writing throwaway code for your own use--it is not necessarily simpler
than managing input and output in any other language.  For example,
robust code should check the state of the input and output streams
between operations (for example, using the method `good').  Note:
Checking the state of a stream.  You may also need to adjust
maximum input or output field widths, using manipulators like `setw' or
`setprecision'.

 - Operator on ostream: <<
     Write output to an open output stream of class `ostream'.  Defined
     by this library on any OBJECT of a C++ primitive type, and on
     other classes of the library.  You can overload the definition for
     any of your own applications' classes.

     Returns a reference to the implied argument `*this' (the open
     stream it writes on), permitting statements like
          cout << "The value of i is " << i << "\n";

 - Operator on istream: >>
     Read input from an open input stream of class `istream'.  Defined
     by this library on primitive numeric, pointer, and string types;
     you can extend the definition for any of your own applications'
     classes.

     Returns a reference to the implied argument `*this' (the open
     stream it reads), permitting multiple inputs in one statement.