Jeffrey Lee Hellrung, Jr. wrote:
+1 for explicit copy-on-write. At that point, you're basically just dealing with something close to an augmented shared_ptr< T const >.
With explicit write(), one could envisage a family of two pointers, one the equivalent of shared_ptr<T const>, the other, returned from write, an equivalent of unique_ptr<T> to the only mutable copy so far. Once you're done writing, you would move it into a read_ptr again. That interface might not prove very convenient in practice though. Although you could, I think, build apply/APPLY upon it. read_ptr<T> pr; if( write_ptr<T> pw = pr.write() ) { pw->mutate(); pr = std::move( pw ); } I don't, however, particularly like the semantics of the case in which pr is already unique. In principle it shouldn't matter because we're writing pr from this thread, so nobody ought to be reading it, but still, it doesn't feel right.