It sounds like you want something built on top of std::vector but with your own rules about when to reserve() and how much.
(Example use case: Some number of objects want to register themselves as observers on some event, and unregister themselves later, in arbitrary order. The same object may register and unregister itself many times. std::unordered_set<Observer*> would be the best fit if performance doesn't matter but an intrusive linked list requires no allocation at all on behalf of the list.)
> I want to be able to dynamically add and remove objects from the list without any allocations at all. The only way to achieve that is an intrusive linked list design
So with that, let me reply to your most recent comment:
> No, for the use cases I'm thinking of, I really do just want an intrusive linked list. Why would I want to invent a convoluted way to adapt std::vector when an intrusive linked list does exactly what I want?
You said an intrusive linked list is the only way to achieve that design. I'm telling you it's not the only way to implement a zero-allocation linked list.
If an intrusive linked list is exactly what you want then great for you. You haven't clearly articulated exactly why you must have an intrusive linked list. I've given you an alternative. You're rejecting it out-of-hand because you don't want to "invent a convoluted way to adapt" already-existing containers.
> Why would I want to invent a convoluted way...
You only think it's convoluted. I argue that it's not convoluted at all.
There are many reasons you would not always be able to use an intrusive linked list. Intrusive linked lists are... intrusive. They require that you:
- directly modify T (again, might not be possible if you don't own the code for T, eg it comes from a third party library)
- or have T as a member (which also might not be possible, particularly whether T must be created by a factory and how convoluted that factory is)
- or else have capability to inherit from T (which might not be possible, see `final` keyword).
You haven't stated any of these (or any other) reasons. You simply stated that an intrusive linked list is the "only" way to achieve zero-allocation linked lists. Without stating why you must have an intrusive list, I have merely offered another option. If you think using the standard library is convoluted here then I wonder what other data structure concepts you will have trouble understanding and suggest that C++ isn't the right language for you.
