Treetree is a header-only C++ library that implements a generic tree-structured container class according to STL conventions, as well as properly overloaded operator>> for input and operator<< for output.

The underlying data structure generalizes a doubly linked list - in addition to previous and next pointers, tree nodes contain a third pointer to the list of their children. The implementation is efficient, and the API is expressive. Treetree is useful for representing anything that one would use S-expressions for in Lisp (inference trees, programs, etc.).

You can iterate over the contents of a tree in pre- or post-order, iterate over the children of a particular node only, or iterate over leaves only. The same options are available for iteration over subtrees (e.g. the subtrees of f(g(x, y) z), in pre-order, are f(g(x, y) z), g(x, y), x, y, and z).