...

I think another alternative would be to use this for counting functionality:

http://www.boost.org/libs/iterator/doc/counting_iterator.html and dumb

...

down your function_input_iterator to be a very simple Incrementable. m.

Any reason to prefer that approach?

Not really, after some pondering I think it would make things harder. Thanks for posting your solution. max. This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This message contains confidential information and is intended only for the individual named. If you are not the named addressee you should not disseminate, distribute or copy this e-mail.