Postconditions: pm == sl_p.pm and owns == sl_p.owns (where
sl_p is the state of sl just prior to this construction),
sl.pm ==nullptr and sl.owns ==false.
Postconditions: pm == sl_p.pm and owns == sl_p.owns (where
sl_p is the state of sl just prior to this assignment),
sl.pm ==nullptr and sl.owns ==false.