Polymorphic copy: Difference between revisions

=={{header|TXR}}==

TXR Lisp has a <code>copy</code> function that produces copies of objects of all sorts.

Structures are shallowly copied; the <code>copy-struct</code> function is used when the

argument is a structure. Our polymorphic object can use <code>copy</code> to make a shallow copy of itself which shares a reference to the contained object. Then break the reference by calling <code>copy</code> on the object contained in the copy.

<syntaxhighlight lang="txrlisp">(defstruct base ()

(:method identify (self) (put-line "base")))

(defstruct derived (base)

(:method identify (self) (put-line "derived")))

(defstruct poly ()

obj

(:method deep-copy (self)

(let ((c (copy self))) ;; make copy of s

(upd c.obj copy) ;; copy self's obj

c))) ;; return c

;; Test

(let* ((b (new base))

(d (new derived))

(p (new poly obj d)))

b.(identify) ;; prints base

d.(identify) ;; prints derived

(let ((c p.(deep-copy)))

p.obj.(identify) ;; prints derived

(prinl (eq p.obj c.obj)))) ;; prints nil: c.obj is not a ref to p.obj</syntaxhighlight>

{{out}}

<pre>base

derived

derived

nil</pre>