CODEWITHSUNDEEP
arrayslispcommon-lisp
jakHunter
jakHunter

Reputation: 315

Lisp why are my objects of the same instance?

So I wanted to remake a program that uses lists for containing objects and replace the lists with arrays.

And I ran into a problem in which my object is of the same instance

(setq arr (make-array 3 :initial-element (make-instance 'object) :adjustable t :fill-pointer 3))

after this code's execution

all of the objects in the array are the same and the interpreter gives me this

#(#<OBJECT {1002DFDD23}> #<OBJECT {1002DFDD23}> #<OBJECT {1002DFDD23}>)

I know other ways to do this but still I want to know why does this happen?

and if it's no trouble what does #<OBJECT {1002DFDD23} mean as a whole and is this some sort memory address?

Upvotes: 0

Views: 175

Answers (3)

Rainer Joswig
Rainer Joswig

Reputation: 139241

Common Lisp HyperSpec: Function MAKE-ARRAY:

If initial-element is supplied, it is used to initialize each element of new-array

Upvotes: 2

Dan Robertson
Dan Robertson

Reputation: 4360

make-array is a function. It’s arguments are evaluated before calling.

The following are equivalent:

(make-array 3
            :initial-element (make-instance 'object)
            :adjustable t
            :fill-pointer 3))

And

(let ((a 3)
      (b :initial-element)
      (c (make-instance 'object))
      (d :adjustable)
      (e t)
      (f :fill-pointer)
      (g 3))
  (make-array a b c d e f g))

Upvotes: 4

Svante
Svante

Reputation: 51501

The form (make-instance 'object) is evaluated exactly once on invocation of your code. The result is used as initial value for each element of the array. It may help to see the singular word initial-element (not -elements), and that operators starting with make- are usually functions and thus all arguments are evaluated before invocation.

The output #<OBJECT {1002DFDD23}> is for an unreadable object. This is indicated by the #< syntax, which is defined to signal an error when trying to read it (http://clhs.lisp.se/Body/02_dht.htm). It is usually produced by print-unreadable-object, which is usually used when defining methods for print-object. The exact output is thus implementation or user defined (whoever wrote a print-object method for that particular class). It will by default most likely contain the class name and something like a memory reference or address.

For completeness, I like to use map-into:

(map-into (make-array 3 :adjustable t :fill-pointer 3)
          (lambda () (make-instance 'object)))

to get an array initialized with distinct objects.

Upvotes: 6

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