Base class for slots.

std::vector<implementation-detail>
bool

`true` if any tracked object has expired.

locked_container_type A container holding `shared_ptr`s to each of the slot's tracked objects. As long as the returned container is kept in scope, none of the slot's tracked objects can expire. `expired_slot` if any of the slot's tracked objects have expired.
bad_weak_ptr
Indicates at least one of a slot's tracked objects has expired.
The `expired_slot` exception is thrown to indicate at least one of a slot's tracked objects has expired.
const char *