Comment by sanderjd

In an ideal world, what you want is more like copy-on-write. Share the code until one of the clients of that code needs a change that the others do not, then copy it. In reality, what happens more often is an accumulation of different functionality used by different clients. Copying "defensively" as you suggest can help avoid that problem, but it has the opposite problem of requiring changes to all copies when all clients want the same thing. In my experience, this is the problem more likely to lead to painful mistakes. There is no silver bullet...