I think everyone will agree that using 4 bytes to store the size a much better idea than potentially enumerating millions of nodes just to count them, in the process completely blowing the cache and any optimizations the processors may do.
It's four goddamn bytes. Or eight if you really want more than 4 billion elements.
Most STL implementations have always done this. This is one reason why it's better to use the empty() check for an empty container instead of size() == 0 for all container types.
Also, linked lists are notoriously crap for cache coherency / processor pre-fetching and branch prediction by processors anyway, given that there's no guarantee where each node will be allocated - the only way to do that is pull them off a slab allocator or something, in which case you might as well use a vector or deque anyway...
That may make 'splice' slower. See http://home.roadrunner.com/~hinnant/On_list_size.html (I think that could be solved by having list iterators carry a field pointing to their container, but haven't given it much thought. Corrections welcome)
Having list iterators carry a field pointing to their container wouldn't help you determine the number of objects between two iterators when you splice, which is what you need in order to maintain the object count.
Storing an index in the iterator would not be a solution either. That would mean updating the index in all the existing iterators when you insert an object in the list.
Just make it the native size based on architecture. 4-byte int for 32-bit, 8-byte for 64-bit. It's impossible to store more than 4 billion elements on a 32-bit machine, you run out of memory first.
That's new in C++11 though, isn't it? I'm pretty sure (i.e. I haven't checked the spec) that as far as C++03 the complexity was allowed to be linear. Honestly my sense of aesthetics agrees -- linked lists aren't supposed to have constant-time operations, there are better choices if that is a requirement. And if you want an augmented data structure you should cook your own: a list with extra tracking (i.e. ever node needs to have a pointer to the global thingy!) and insertion hooks isn't a "list" any more.
Yeah, they changed the wording from "should" to "must" be O(1) in C++11, which means that before, even when they encourage the implementer to do it constant, that was in fact optional.
I don't that tracking the size would be a big deal, if you think that doing that they wouldn't be a linked list any more, you could think it as a linked link wrapper.
In any case the complexity guarantees of all the other operations remain true so I don't think it is a big deal.
(libc++ keeps a track of the size somewhere, but this in occurs a 4/8 byte overhead)