Why can't I assign one stream to another?
=========================================

   [ Thanks to Per Bothner and Jerry Schwarz for this section. ]

   Assigning one stream to another seems like a reasonable thing to do,
but it's a bad idea.  Usually, this comes up because people want to
assign to `cout'.  This is poor style, especially for libraries, and is
contrary to good object-oriented design.  (Libraries that write directly
to `cout' are less flexible, modular, and object-oriented).

   The iostream classes do not allow assigning to arbitrary streams,
because this can violate typing:

     ifstream foo ("foo");
     istrstream str(...);
     foo = str;
     foo->close ();  /* Oops! Not defined for istrstream! */

   The original cfront implementation of iostreams by Jerry Schwarz
allows you to assign to `cin', `cout', `cerr', and `clog', but this is
not part of the draft standard for iostreams and generally isn't
considered a good idea, so standard-conforming code shouldn't use this
technique.

   The GNU implementation of iostream did not support assigning to
`cin', `cout', `cerr', and `clog' for quite a while, but it now does,
for backward compatibility with cfront iostream (versions 2.6.1 and
later of libg++).

   The ANSI/ISO C++ Working Paper does provide ways of changing the
streambuf associated with a stream.  Assignment isn't allowed; there is
an explicit named member that must be used.

   However, it is not wise to do this, and the results are confusing.
For example: `fstream::rdbuf' is supposed to return the *original*
filebuf, not the one you assigned. (This is not yet implemented in GNU
iostream.)  This must be so because `fstream::rdbuf' is defined to
return a `filebuf *'.