A slot specifier that includes options such as :initarg or :initform is written as a list starting with the name of the slot followed by the options. For example, if you want to modify the definition of bank-account to allow callers of **MAKE-INSTANCE** to pass the customer name and the initial balance and to provide a default value of zero dollars for the balance, you’d write this:

    Now you can create an account and specify the slot values at the same time.

    1. (defparameter *account*
    2.   (make-instance 'bank-account :customer-name "John Doe" :balance 1000))
    4. (slot-value *account* 'customer-name) ==> "John Doe"
    5. (slot-value *account* 'balance)       ==> 1000

    If you want to ensure that the customer name is supplied when the account is created, you can signal an error in the initform since it will be evaluated only if an initarg isn’t supplied. You can also use initforms that generate a different value each time they’re evaluated—the initform is evaluated anew for each object. To experiment with these techniques, you can modify the customer-name slot specifier and add a new slot, account-number, that’s initialized with the value of an ever-increasing counter.

    1. (defvar *account-numbers* 0)
    3. (defclass bank-account ()
    4.   ((customer-name
    5.     :initform (error "Must supply a customer name."))
    6.    (balance
    7.     :initarg :balance
    8.     :initform 0)
    9.    (account-number
    10.     :initform (incf *account-numbers*))))

    Most of the time the combination of :initarg and :initform options will be sufficient to properly initialize an object. However, while an initform can be any Lisp expression, it has no access to the object being initialized, so it can’t initialize one slot based on the value of another. For that you need to define a method on the generic function **INITIALIZE-INSTANCE**.

    Then you can define an :after method on **INITIALIZE-INSTANCE** that sets the account-type slot based on the value that has been stored in the balance slot.7

    2.   (let ((balance (slot-value account 'balance)))
    3.     (setf (slot-value account 'account-type)
    4.             ((>= balance 100000) :gold)
    5.             ((>= balance 50000) :silver)
    6.             (t :bronze)))))

    The **&key** in the parameter list is required to keep the method’s parameter list congruent with the generic function’s—the parameter list specified for the **INITIALIZE-INSTANCE** generic function includes **&key** in order to allow individual methods to supply their own keyword parameters but doesn’t require any particular ones. Thus, every method must specify **&key** even if it doesn’t specify any **&key** parameters.

    By defining this **INITIALIZE-INSTANCE** method, you make :opening-bonus-percentage a legal argument to **MAKE-INSTANCE** when creating a bank-account object.

    1. CL-USER> (defparameter *acct* (make-instance
    2.                                 'bank-account
    3.                                  :customer-name "Sally Sue"
    4.                                  :balance 1000
    5.                                  :opening-bonus-percentage 5))
    6. *ACCT*