A copy-on-write string is pretty similar to a shared_ptr to char. Like a shared_ptr, a shared string has to check whether it is the last referent when it goes out of scope. This introduces a point of contention on multi-core systems. A string like the one described in this article might not have to do anything in its destructor, whereas a shared string will need to atomically decrement a reference count. That’s a huge difference in cost.

The benefit of a shared string is you might save some space. That’s fine but I think most programmers today will choose am explicitly shared type when they need it, and a fast string for most cases.