From: "Howard Hinnant"
On Apr 25, 2004, at 9:29 PM, Peter Dimov wrote:
In fact, it isn't even clear to me why you need to synchronize access to the implementation at all, as std::locale is immutable; it has no non-const member functions.
Ah, I think I see the confusion. locale is mutable. The copy ctor mutates the rhs (though not visibly) and the assignment mutates both sides. There's more refcounting running around under locale's hood than just the shared_ptr to the implementation. In addition each facet is individually refcounted (but not with shared_ptr). Even std::use_facet potentially mutates a locale (via lazy addition of a standard facet to the locale).
Yes, I see. It isn't very common for a class to physically (but not logically) mutate the rhs on copy. ;-) But I agree that in this case you'd gain nothing from separate count synchronization. Let's go a bit further. The obvious alternative to shared_ptr here, to me, is not a non-synchronized shared_ptr, but an embedded reference count (no need for separate count synchronization -> no need for separate count). Is there something else that I'm missing that makes intrusive counting unsuitable?