Rope Implementation Overview
The rope container type included in SGI's version of the STL is based loosely on the ropes in the Xerox Cedar environment or C "cords", as described in Boehm, Atkinson, and Plass, "Ropes: An Alternative to Strings", Software Practice and Experience 25, 12 (Dec 1995), pp. 1315–1330.
A rope is represented as a pointer to _Rope_RopeRep structure, which represents a tree node. Every tree node corresponds to a piece of a rope. Although we refer to "tree nodes", each such piece can be shared between different ropes, or can even be reused in the same rope if the corresponding substring is repeated. Thus ropes are really represented as directed acyclic graphs. Nonetheless, we will continue to refer to trees, since that is both the usual case, and more intuitive.
Each tree node contains a size field giving the length of the rope piece, a depth field specifying the depth (or height) of the tree rooted at the node, a boolean field indicating whether the subtree has been balanced, and a tag field indicating which of the four variants or subclasses of _Rope_RopeRep is used to represent the list. (The balanced bit is really of interest only for concatenation tree nodes, see below.)
It would have been possible to use virtual functions and/or RTTI to replace the use of the tag field. We chose not to pursue that route, since the tag field can be much smaller than a vtable pointer, and the tag based code is probably also faster in this case.
The 4 subclasses of _Rope_RopeRep are:
1. (_Rope_RopeLeaf) Leaves containing string characters. Short ropes are usually represented as a single such node. In the case of the standard character type, the actual array of characters is NULL-terminated to allow fast generation of an equivalent C string.
2. (_Rope_RopeConcatenation) Concatenation nodes. These have two children left and right. They represent the concatenation of the two strings represented by the left and right subtrees. Concatenation of two longer ropes usually allocates a new concatenation node which references the two ropes to be concatenated.
3. (_Rope_RopeFunction) Function nodes. These contain a pointer to a function object that can be used to compute sections of the string. This facility makes it possible to manipulate a rope that is computed lazily as the pieces are needed. For example, it is possible to treat a file as a rope without actually reading in the entire file. Thus a text editor can represent even a 100 MB file being edited as a rope, updating it with standard rope operations, while still consuming only very small amount of memory.
4. (_Rope_RopeSubstring) Substring nodes. These contain a pointer to a base rope tree node, and a starting position within that rope. They denote a substring of the base rope. These are generated only to represent substrings of ropes that are expensive to compute explicitly. The base field never points to a concatenation tree node. If the substring operation is applied to either a very large leaf node (which can be built by converting a very long C string to a rope) or to a function node representing a long string, then it produces a substring node. Substring nodes also contain a pointer to a function object that performs the appropriate character extraction. They are a subclass of function nodes, and a number of operations treat them simply as function nodes. Many uses of ropes will never result in the generation of a substring node. They are however essential for applications that use function nodes to lazily evaluate strings.
Only concatenation nodes have nonzero depth fields. Depth fields are guaranteed to fit into a byte, since we impose a static maximum on rope depth.