I will reply to people in series below:

Marcin Jaczewski wrote:
>
> No, standard should not allow this, reason is simple
> you hide multiple indirections that some could be `nullptr` or empty:
> ```
> std::optional< std::optional<std::string>* > p { nullptr };
> if (p) p->size(); // 2x UB and even if check look correct
> (***p).size(); //easy to see that you need 3 checks


In my code I check that the std::optional object contains a value, and
if it does then the logic of my code is that the iterator is always
valid.

But anyway maybe we could have a new operator such as '->>' which
stops when it encounters a nullptr. For example if you do:

    (***p).size(); // undefined behaviour
    p->>size(); // nothing happens


Giuseppe D'Angelo wrote:
> If it's the former, if you actually use any container that yields
> forward iterators, you can just use a value-initialized iterator object
> as "null iterator" https://eel.is/c++draft/forward.iterators#2 . Just
> static_assert that your iterator models forward_iterator.
>
> If it's the latter, then there's nothing to worry about because
> std::list never invalidates its end() iterator.


I want the code to be generic so that the container can be changed
later. I might end up using
boost::lockfree::single_producer_single_consumer.