On 3/6/2011 5:01 PM, Peter Dimov wrote:
Chad Nelson wrote:
The only concrete complaint that people could give me about CoW was that code using it couldn't be made thread-safe, which is true.
Can you please provide more details? Why is not CoW thread safe?
I *can* be made thread-safe if you use atomic operations (as far as I know; someone correct me if this I'm wrong here). However, that likely introduces overhead even when you only need single threaded usage. If you don't use atomic operations, then you have problems when multiple threads try to fiddle with the reference count, leading to the potential for dangling memory blocks and/or double deletions. COW can introduce a performance hit with every in-place modifying operation, as you have to ensure sole ownership (so it requires a possible branch; you can't count the deep copying, of course, as that would happen anyway without COW). I don't know how many operations in *this* library fit that category, however; enough to justify the existence of a _make_unique member function? I would also expect this overhead to be negligible to the usual arithmetic operations once your integers reached a certain size... - Jeff