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copy-structure

copy-structure Function

Syntax:

copy-structure structure → copy

copy-structure

Arguments and Values:

structure—a structure.

copy—a copy of the structure.

Description:

Returns a copy6 of the structure.

Only the structure itself is copied; not the values of the slots.

See Also:

the :copier option to defstruct

Notes:

The copy is the same as the given structure under equalp, but not under equal.

Expanded Reference: copy-structure​

Basic Usage​

copy-structure creates a shallow copy of a structure. The copy has the same type and slot values as the original, but is a distinct object.

(defstruct point x y)
=> POINT

(setq p1 (make-point :x 1 :y 2))
;; => #S(POINT :X 1 :Y 2)

(setq p2 (copy-structure p1))
;; => #S(POINT :X 1 :Y 2)

(point-x p2)
=> 1

(point-y p2)
=> 2

The Copy Is a Distinct Object​

(eq p1 p2)
;; => NIL

(equalp p1 p2)
;; => T

Modifying the Copy Does Not Affect the Original​

(setf (point-x p2) 99)
;; => 99

(point-x p1)
;; => 1

(point-x p2)
;; => 99

Shallow Copy: Shared Slot Values​

The copy shares the same slot value objects -- it is not a deep copy.

(defstruct container items)
=> CONTAINER

(setq c1 (make-container :items (list 1 2 3)))
;; => #S(CONTAINER :ITEMS (1 2 3))

(setq c2 (copy-structure c1))

(eq (container-items c1) (container-items c2))
=> T

(push 0 (container-items c2))
;; push creates a new cons in c2's slot; c1's slot still points to the old list:
(container-items c1)
=> (1 2 3)

Equivalent to the Auto-Generated Copier​

Each defstruct definition creates a copier function (e.g., copy-point). For a structure type without :type, copy-point is equivalent to copy-structure.

(equalp (copy-point p1) (copy-structure p1))
;; => T