Hi Nicholas The final design is now waiting on Dan, but it is always interesting to see other ideas.
Like you, I rejected the parent/child technique. However, my proposed solution did not use any links at all, because it relies on the garbage collection system to determine when a shared buffer has only one reference. It worked as follows (ignoring substrings for simplicity of explanation): Each string header (the equivalent of a scalar in this regard, as it points to a specific string instance) points to the shared buffer The string header contains a COW flag, set in both source and destination headers when a string is [not] copied The buffer also contains a flag, used only during garbage collection and always initialised to zero When a string needs to be changed, we simply copy the data into a new buffer and clear the COW flag in the header During the memory pool compaction phase of the GC system, the first header encountered that points to a specific buffer will cause the buffer to be collected as normal; the special flag is set in the old copy of the buffer to say that this buffer has been relocated, and the address of this first header is stored in the old buffer. The COW flag is cleared at this time. If another header is found that points to a buffer that has already been moved, no copying takes place, and the header is simply corrected to point to the new location of the buffer. The first header has its COW flag set again. The result is that the last header of a COWed string will still believe that the buffer is shared until a GC collection run occurs, and therefore could result in buffers being copied unnecessarily. Your system eliminates this problem; however, I believe that Dan may be averse to using a linked list - we'll see. -- Peter Gibbs EmKel Systems
