Sandeep,

that with storage depends on how you are going to solve your problem. You might use boost::iterator_adaptor to put an iterator ontop of your ForwardIterator range and deal with iteratos only. That might be cheaper instead of createing set of value_types create a set of iterators. Overloading your own less then operator might compare boost::iterator_adaper-specialization instances which are than stored in std::set. Set will only contain iterators and not real object instances. This assumes that your input range lives long enough.

Or you can intorduce a filter_iterator which might implement a sort of lazy unique iterator. So it might contain a set with objects already iterated though and decide whether to skip to the next element when incremented or not.

Hope that helps,
Ovanes