Hi all, Over the past few weeks I've been slowly working on (what I hope to be) a candidate for a boost::tree. I've reviewed the comments on previous submissions by Kasper Peeters (http://www.damtp.cam.ac.uk/user/kp229/tree) and Justin Gottschlich (http://nodeka.com/TreePart1.pdf), and I'm trying to put together a set of concepts to address some of the issues that were discussed here. Of the two "how to design a generic tree container" camps, I'm finding myself mostly in the one where individual nodes are not exposed to users, and the tree is an opaque structure. Also, nodes are not identical to iterators, and the iterator concept is not extended in any significant way from the standard. (Anybody who would like more detail, I'm happy to provide my draft of the concepts, but I don't want to get into that detail here -- it's not particularly relevant to my questions that follow.) I feel like I've been making decent progress so far, mostly working on paper, just documenting the concepts, and doing some coding exercises for proofs of concept. But I don't have anything I'd consider ready for primetime yet -- not even remotely -- and in fact as I progress more solidly into the coding phase, I'm finding myself getting somewhat mired in the details. So I thought I'd solicit opinions from experienced Boosters here on some of the issues I'm trying to work through. I'm primarily finding myself conflicted about what degree of flexibility the concepts and their concrete implementations need to offer. I thought it would be useful to allow clients of tree<> to be able to specify the container type for a node's children. In the code I'm currently playing with, I have a couple "selector" structs, basically identical to Boost Graph Library's vecS, listS, etc, and the tree<> class template takes one of these selectors as a template parameter. That much is all well and good, but I quickly run into trouble once I start dealing with nodes and iterators. First of all, I'd like to provide access to the children of a given node as a sequence of the type specified by the client. So if you have an iterator to an element in the tree (the structure of the tree being otherwise opaque), I want to be able to say something like: tree<foo, list_selector> t; tree<foo, list_selector>::some_kind_of_iterator i; // assume t contains something useful and i points to an element in t std::list<foo> children = t.children(i); At this point, I've got a problem: internal to the tree<>, the lists representing child sequences contain not instances of type "foo" but instances of something like node<foo>. So I can't simply provide access like this unless I can guarantee that my nodes are convertible in all contexts to their stored data. That seems like a rat hole. Another thing I'm stuck on is, even if I don't expose the container structure like this, I still have a problem with iterators. The above example omits the actual iterator type, but to get into some specifics, one type of iterator I'd like to provide is a pre-order traversal iterator. In my current code, this iterator is basically just a wrapper for iterators into the flat sequences I've been instructed to use, and some minimal code that knows how to jump between these sequences appropriately (for example: if a node has children, operator++ yields the first child of that node). It's easy to get an iterator into the sequence containing a node's children (internally, it's just something like node.children.begin()), but it's not so easy to get an iterator into the sequence containing a node's parent, unless a node contains not a pointer to its parent but an iterator instead. And that scheme breaks down for children of the root node, which is not in any sequence, as it has no siblings. I'm sure there are other options for how to implement the node structure and iterator relationships, but none jump out at me. In short, although I'd like my concepts for trees to allow users to specify and access the sequences representing the children of a node, my implementation of this is proving very clumsy. So I'm wondering two things: 1. Is this a good idea in the first place? Are there really any compelling use cases, where you'd want one tree to store its node structure in vectors and another in lists? Or is this a case where a policy-based approach is misplaced effort? 2. If this actually is a good idea (which I'm starting to get the feeling is not the case), my experience so far is that there's some significant overhead -- both in terms of the size of the structures, and in terms of speed (extra pointer traversals when doing anything with iterators). Does anybody have any brilliant ideas about how to minimize this overhead? If these questions are too vague, or lacking in context, I'm more than willing to provide the rough draft I've got of the structural concepts, and the very incomplete code (such as it is). Thanks in advance for your expert advice! dr