I think there are several reasons why there are no STL trees. Primarily Trees are a form of recursive data structure which, like a container (list, vector, set), has very different fine structure which makes the correct choices tricky. They are also very easy to construct in basic form using the STL.

A finite rooted tree can be thought of as a container which has a value or payload, say an instance of a class A and, a possibly empty collection of rooted (sub) trees; trees with empty collection of subtrees are thought of as leaves.

template<class A> struct unordered_tree : std::set<unordered_tree>, A {}; template<class A> struct b_tree : std::vector<b_tree>, A {}; template<class A> struct planar_tree : std::list<planar_tree>, A {};

One has to think a little about iterator design etc. and which product and co-product operations one allows to define and be efficient between trees - and the original STL has to be well written - so that the empty set, vector or list container is really empty of any payload in the default case.

Trees play an essential role in many mathematical structures (see the classical papers of Butcher, Grossman and Larsen; also the papers of Connes and Kriemer for examples of they can be joined, and how they are used to enumerate). It is not correct to think their role is simply to facilitate certain other operations. Rather they facilitate those tasks because of their fundamental role as a data structure.

However, in addition to trees there are also "co-trees"; the trees above all have the property that if you delete the root you delete everything.

Consider iterators on the tree, probably they would be realised as a simple stack of iterators, to a node, and to its parent, ... up to the root.

template<class TREE> struct node_iterator : std::stack<TREE::iterator>{ operator*() {return *back();} ...};

However, you can have as many as you like; collectively they form a "tree" but where all the arrows flow in the direction toward the root, this co-tree can be iterated through iterators towards the trivial iterator and root; however it cannot be navigated across or down (the other iterators are not known to it) nor can the ensemble of iterators be deleted except by keeping track of all the instances.

Trees are incredibly useful, they have a lot of structure, this makes it a serious challenge to get the definitively correct approach. In my view this is why they are not implemented in the STL. Moreover, in the past, I have seen people get religious and find the idea of a type of container containing instances of its own type challenging - but they have to face it - that is what a tree type represents - it is a node containing a possibly empty collection of (smaller) trees. The current language permits it without challenge providing the default constructor for container<B> does not allocate space on the heap (or anywhere else) for an B , etc.

I for one would be pleased if this did, in a good form, find its way into the standard.