"istream" problem

[Aaron Gray]

I need an input stream that can be cloned and assigned.

I need to be able clone the stream, read from the clone, and if that read
was consided "correct input" then to update the original stream to the
cloned stream.

pseudo code:

stringbuf sb( "test input");
istream i( sb);
istream ic( i);

if ( ic.compare("test"))
i = ic;

Can I do a simular thing with C++'s istream or do I need to write my own
istream class ?

Aaron