Thanks! (On mobile, so bear with me) On the encapsulation PR, kaz suggested dropping the std::function arg in favor of a templated Callable in the header, to avoid incurring the stl overhead for simple lambdas. I think that's a good idea. Mind doing that at the same time here? So It'd look something like (all in the header) template <typename Callable> void ForEachNode(Callable&& func) { LOCK.. for(nodes...){ func(node); } } Also, if it would end up being simpler, you could implement them all terms of a basic (private, possibly static) version with no locking. With that, I believe it's also possible to eliminate ForEachNodeThen in favor of passing in a callable object, with its "then" function simply defined as its destructor. I'm not looking at the code now though, so i could be wrong on that.