Very impressive work! That took quite some sleuthing.

I wonder, however, of instead we should delete operator& from all of our CoWing types (Vector, String, etc), reason being that by taking a pointer to the container you circumvent CoW, which is what, in my mind, lies at the root of this.

I think with a CoW datatype taking a pointer to it should be an error, because you should never be storing a reference to the container, you should always take a copy and rely on CoW to make it safe.

If a piece of code can't do that, then it shouldn't be using CoW to begin with. Especially now with the move operators, I feel that if you need to do processing on a CoW type to later pass it on, it is only marginally more expensive to just create a LocalVector to then later move it to a Vector for passing on by using operator Vector<T>.

I have changed my mind a little bit, I think that what I wrote above is true, it is also kind of off topic.

When refcount == 0, the only valid state for ptr() is nullptr. Anything else would be a violation of the contract.

EDIT: that is to say you are right. :)

Thanks!