Return a "snapshot" of the number of shared pointers (including this one) that share ownership of the object managed by this shared pointer. Note that 0 is returned if this shared pointer is empty. Also note that any result other than 0 may be unreliable in a multi‐threaded program, where another pointer sharing ownership in a different thread may be copied or destroyed, or a weak pointer may be locked in the case that 1 is returned (that would otherwise indicate unique ownership).