How to manage lifetime of the type "shortest of"?

We use composition, when an object has a single parent, which should care for the object's lifetime. We use unique_ptr when in the same situation, but the object can be nullptr.

We use shared_ptr when several external entities may need our object, so its lifetime is extended until the last of those external entities loses interest.

Here I want to ask about another lifetime situation. What if the object needs to live the shortest of several durations?

Here is an example. Let's have a one-shot timer, which stores a functor and executes it after a the counting is complete. It makes sense to me*, that this timer object is destroyed after:

1. fulfilling its task - therefore it should be able to destroy istelf

 or

2. the parent loosing interest in the timer - so the parent should be able to 
     destroy the object as well

Currently, I using an awkward implementation with unique pointers. What would be a good pattern / guideline / implementation of this problem?

_{* reasons: 1) the functor could be owning some other resources 2) the timer could have been set to a very large number, and then abandoned 3) if the parent has been destroyed, we generally don't want to invoke its callbacks}