Hi Rutger,
I think I know what optional models, and it is clearly what I want. However, I would like to have space-efficient ones.
Achieving the space-efficiency via specific values is not an option for the reasons stated before. On the other hand, doing that by some other means is always worth consideration. That has also been discussed in the past, but sadly I don't remember any conclusion. Off the top of my head (and past midnight) I'm thinking that a specialized version of aligned storage that always allocate one extra byte at the end might do the trick. Having said that, there is a problem with your *requirement*. I have had sort of the opposite: replace the bool with a pointer to T, for debugging purposes (out-of-the-box pretty printing). The justification being precisely that the bool takes up the same space anyway. I really liked that change, and I was planing on doing it. Now I'm not so sure, because your's is also valid (if no much more). -- --- Fernando Cacciola SciSoft Consulting, Founder http://www.scisoft-consulting.com