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4 points by kennytilton 6126 days ago | link | parent

In AllegroCL:

  (defun copy-instance (i &aux (i-class (class-of i)))
   (let ((c (make-instance i-class)))
    (loop for s in (mop:class-slots i-class)
        do (setf (slot-value c (mop:slot-definition-name s))
             (slot-value i (mop:slot-definition-name s)))
          finally (return c))))
Where you see mop: look out. :) The MOP package of the CL implementation you are using may even be nicknamed CLOS. You may have to take the union of class-direct-slots and class-indirect-slots. The slot returned may be the slot name itself. Stuff like that.


3 points by sross 6125 days ago | link

Although I would suggest using allocate-instance over make-instance and ensuring that

  (slot-boundp i (mop:slot-definition-name s))
is true before attempting to obtain the slot-value.

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3 points by kennytilton 6125 days ago | link

OK, that (and crankin the loop) would make it:

  (defun copy-instance (i)
     (loop with i-class = (class-of i)
      with c = (allocate-instance i-class)
      for sd in (mop:class-slots i-class)
      for sn = (mop:slot-definition-name sd)
      when (slot-boundp i sn)
      do (setf (slot-value c sn)
           (slot-value i sn))
      finally (return c)))
But what if necessary side-effects happen in initialize-instance? Though that could as easily make a case for allocate-instance, if Paul needs to avoid those side effects.

I think the bottom line is that I should have emphasized that copy-instance above might not survive first contact with the enemy (the functional requirements).

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