The Standard Template Library (STL) supplies C++ developers with many useful generic container classes. One type of container not found in the STL the tree container. The Tree Container Library (TCL), presented here, is a generic C++ tree container library which, not only works much like the STL, but is compatible with the STL algorithms and other STL containers. The latest version of the TCL can be downloaded from the TCL download page. The library and all examples are compatible with VC6, VC7, and VC8 (Visual Studio 2005), as well as gcc. Most other C++ compilers should have no problems with the library, if they are compliant with C++ template standards.
The TCL consists of four template container classes, similar to those found in the STL. These classes allow storage of basic data types or user defined types to be stored within nodes in a tree structure. Each node of the tree is considered a tree itself, having all the properties of the tree container that it's a part of. So, all operations which can be performed on the tree container can likewise be performed on any node within the tree. Iterators are provided to allow the traversal of the tree nodes. Insert and find operations, as well as various other operations, are also provided.
The TCL provides four tree containers which differ according to their intention of use:
The tree and multitree are very similar in operation and interface. The difference between the two is much like the
difference between the set and multiset in the STL. The unique_tree offers many more features, since each node in the tree is unique.
For example, the
find() operation is available to tree, multitree, and unique_tree, which searches for
a matching child node contained within a single parent node. The unique_tree offers an additional operation,
find_deep() which not
only searches the parent node's immediate children, but also
searches it's descendants.
The unique_tree also offers extensions to the common interface for the three tree types. All four trees offer the
operation, in which a child is inserted in the parent issuing the insert operation. The unique_tree, however, offers an extension to this and
other operations. For example, the unique_tree provides another insert operation,
insert(parent, child), which inserts the child
in the specified parent node (if found).
tree, multitree, and unique_tree are considered associative tree containers, since they all use std::set for internal child node containment. sequential_tree, however, uses a std::vector to store the child nodes. Thus, sequential_tree does not offer find() operations like the associative tree containers do. sequential_tree has it's own unique operations, however, like multiple sort() operations which can be used to sort any/all of the child nodes within their parent. The associative trees do not need sort() operations, because their nodes are ordered naturally.
To understand the structure of the tree containers in the TCL, and how the iterators work, a good understanding is needed for the concept of a node and an element. In the TCL, the term node is used to refer to an object in a tree structure which contains the stored_type element. The stored type element (elements of which you are storing in the tree) is not considered the node itself, but is contained within the node. The tree structure is, then made up of nodes, with the top node being the root node. All nodes in the tree structure (with a couple exceptions) have the same operations and properties of the tree itself. The concept of an element is to specifiy the stored_type object which is being stored within the nodes of the tree. The node and element are considered as two seperate entities. The tree structure consists of nodes. Nodes can likewise have child nodes, and those can also have their own child nodes. The elements reside in the nodes, and are the objects which the user has specified to store in the tree.
Like the STL, the TCL uses iterators in many of it’s operations. TCL iterators can be catagorized in multiple ways. For all iterators,
there are the const and non-const varieties. Another way TCL iterators can be categorized, is by their traversals. child iterators are the most common type of iterators in the TCL, and
traverse only the immediate children of a given parent node. There are also child reverse iterators available, which traverse in a
reverse direction over the immediate children of a parent node. For traversing both breadth and depth of a tree container, there are
descendant iterators available, for pre-order,
post-order, and level-order traversing. The
unique_tree also offers a special ordered_iterator to traverse
the child nodes in an alternately defined order. The final way in which the TCL iterators can be categorized is by what they expose. In this
respect, there are two types of iterators available, the element
iterators, more commonly refered to as just iterators, and the node iterators. The element iterators expose the underlying element, while the node iterators
expose the underlying node. Normally, the most common type of iterators used in the TCL are the element iterators. In all discussions in
the TCL documentation, if the distinction is not made to element iterators or node iterators, then the element iterators
is assumed. When the term iterator is used in the documentation, it could be used to represent all iterator types, only child
iterators, or only child element iterators, depending on the context.
Not only can iterators be used to traverse the tree structures in various ways, many of the operations performed on the trees return an
iterator, such as the find() and insert() operations. These iterators that are returned from many of the TCL tree container operations are
normally chiild element iterators, unless clearly named to specify otherwise. All iterators are created using the same syntax as
iterators in the STL. If a tree container contains objects of the type CMyClass, a child element iterator would be created as
tree<CMyClass>::iterator it;, or
tree<CMyClass>::const_iterator it;. These two child iterators traverse
only a parents immediate children.
To traverse a parents descendants as well as it’s children, there are 12 descendant iterators provided,
level_order_iterator, and their node iterator counterparts,
level_order_node_iterator. Each of these six
iterators also have a const variety. The creation of these descendant iterators uses the same syntax as the child iterators mentioned
Both child and descendant iterators can be used with the STL algorithms, as well as both element and node
iterators. (see STL Compatibility.)
This means that the elements, which reside in the nodes of the tree containers can be copied back and forth between containers in the STL using the element iterators. The nodes can be copied back and forth between containers in the STL using the node iterators. Also, virtually any STL algorithm can now be used with TCL tree containers and iterators. When used with the STL algorithms, it must be clear that the TCL element iterators expose the node's elements, while the node iterators expose the nodes.
As mentioned above, for the element iterators, the * and -> operators are overridden to return the reference/pointer to the
underlying element within the node to which the iterator points. All element iterators also have a
which returns a pointer to the underlying node which contains the element. By using the element iterator's
node() operation, the
node's operations, such as
find() are available to the element iterator. Remember that nodes have the
same interface as the declared tree in which the nodes reside. (Nodes are themselves trees, or actually subtrees). The only difference between
a declared tree container and one of it's nodes, is the simple fact that the declared tree container doesn't have a parent node. (It's the root
I would like to give some credit for the success of this library to a couple of my past comrades, Brock Peabody and Mike Pace, for introducing me to the world of STL, templates, and C++ generic programming. Also thanks to those who have given me invaluable help and suggestions, in which some are mentioned in the credits.