To share ownership between unrelated `thread`s use `shared_ptr`

Reason

If threads are unrelated (that is, not known to be in the same scope or one within the lifetime of the other) and they need to share free store memory that needs to be deleted, a shared_ptr (or equivalent) is the only safe way to ensure proper deletion.

Example

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Note

• A static object (e.g. a global) can be shared because it is not owned in the sense that some thread is responsible for its deletion.
• An object on free store that is never to be deleted can be shared.
• An object owned by one thread can be safely shared with another as long as that second thread doesn't outlive the owner.

Enforcement

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