Hello, Say a library programmer wants to make a virtual interface to his class with a function that returns a collection of things. Usually one end up writing something silly like class interface { ... virtual std::vector< ... > give_stuff() const; }; which has some disadvantages, the most important being: - The implementing function must calculate the whole result, even if some of it might not be needed. Especially infinite collections of things are not possible in this way. - The vector is a container and that's a poorly defined concept. As a consequence, we can't do very much with it. For instance, you can't write give_stuff() + give_more_stuff() to concatenate. Though it's easy to implement that, we really don't want to do this on the basis of containers in general and we also don't want to do it again and again in every single project for std::vector in particular. - As the library gets used, it's poor interface is carved in stone, and so are the other two problems for it. The first problem can be addressed by using an iterator to some container. We obviouly can't use an iterator to a vector since that wouldn't solve any problems. It has to be a custom iterator class lib_iterator_impl : ... { ... virtual ... dereference() const; virtual ... advance() const; }; class lib_iterator { ... lib_iterator(lib_iterator_impl const* impl); }; class interface { ... virtual lib_iterator give_stuff() const; }; where lib_iterator_impl than has to be extended by the client of the library. Actually, there is no real need for lib_iterator to be a proper iterator in the standard sense. Especially, such iterators could be aware of there ends (or even their beginnings) to make it unnecessary to have always two of them. Both the virtualisation and halving the number of those iterators would also reduce the complexity of error messages. On such a framework, many small building blocks could be build, like concatenation, filtering, mapping, cartesian products, etc. In my opinion, at least the very low-level parts of such a framework are so important that they belong in boost. My question at this point is in what regard people share this opinion and to what extend we already have these things. I have heard of any_iterator in the past, but I can't find anything about it in the boost malings list archives or google, which would be related to this. Other than this, I'm not aware of anything in this regard. Cheers, Jens