How do I access a class object stored in an array?

Question

So If I have a defclass object and I make an instance of it and place it inside an array. How do I get the value of its slots inside the array?

I've tried:

(slot-value (aref *array* 0) :name)

I guess I am just not understanding how to access an object that is inside an array.

I can print the object in an unreadable form using (format t) but is there a way to print an object and all the slots in a form I can actually understand?

(defun generate-object (name)
  (let ((a (make-instance 'person
                          :name name)))
    (setf (aref *array* 0) a)))

it places the object inside the array but it seems that the slot is not being created?

This causes the problem:

(defclass person ()
  ((name :accessor name
         :reader read-name
         :initarg :name)))

(defvar *array* 0)
(setf *array* (make-array 20))

(defun generate-object (name)
  (let ((a (make-instance 'person
                          :name name)))
    (setf (aref *array* 0) a)))

I need to access the value of a slot inside an object that is inside an array. I don't know how to do it. — Cody Mallery
– Cody Mallery, Commented Mar 10, 2014 at 17:20
I get an error that says: when attempting to ~A the slot ~S is missing from the object. but my function creates the class and sets the values of the slots correctly. I'll edit my function into my post. — Cody Mallery
– Cody Mallery, Commented Mar 10, 2014 at 17:27
Could you post a complete but minimal version of your code that exhibits the problem? — sigjuice
– sigjuice, Commented Mar 10, 2014 at 17:40

sigjuice · Accepted Answer · 2014-03-10 20:33:35Z

3

The slot name needs to be a symbol that is syntactically valid as a variable name. Try 'name instead of :name .

(slot-value (aref *array* 0) 'name)

Look at the examples here.

edited Mar 10, 2014 at 20:33

answered Mar 10, 2014 at 18:12

sigjuice

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3 Comments

Joshua Taylor Over a year ago

That's the same syntax as the original: slot-value object slot-name. The important difference is that OP's object didn't have a slot named by the keyword symbol :name, but rather by some other symbol name.

sigjuice Over a year ago

@JoshuaTaylor I have fixed my sloppy wording. Also, according to the standard, it looks like slot names cannot be keyword symbols.

Joshua Taylor Over a year ago

+1 for updating. The answer was definitely what was needed; I was just nitpicking. :) Investigating keyword use is always a good idea. I've always liked that you can use non-keyword symbols as keyword-names for keyword arguments. E.g., (defun foo (&key ((bar baz))) baz) and call (foo 'bar 42).

Svante · Accepted Answer · 2014-03-10 21:09:53Z

While possible, it is not recommended to use slot-value outside of the low-level class-specific code (like initialize-instance methods etc.).

You should instead add accessors to your slots and use those. For example:

(defclass foo ()
  ((bar :reader foo-bar
        :initarg :bar)))

This defines a class foo with a slot bar. You can initialize the slot upon object instantiation with the :initarg name:

(let ((my-foo (make-instance 'foo :bar "baz")))
  #| whatever |#)

You can read the slot value with the defined :reader:

(let ((my-foo (make-instance 'foo :bar "baz")))
  (foo-bar my-foo))

It doesn't matter where you get your foo from, of course. Imagine you have an array foo-array which is filled with foos. To get the bar slot value of the fourth foo in that array:

(foo-bar (aref foo-array 3))

If you also want to set the value, use an :accessor instead of the :reader slot option.
Then you can use it as a place:

(let ((my-foo (make-instance 'foo)))
  (setf (foo-bar my-foo) "quux"))

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