The reference to size in the specification of data works the way you’d expect because the expressions that read and write the data slot are wrapped in a **WITH-SLOTS** that lists all the object’s slots. However, if you try to split that class into two classes like this:

(define-binary-class frame ()
  ((id (iso-8859-1-string :length 3))
   (size u3)))
  ((data (raw-bytes :bytes size))))

you’ll get a compile-time warning when you compile the generic-frame definition and a runtime error when you try to use it because there will be no lexically apparent variable size in the read-object and write-object methods specialized on generic-frame.

What you need to do is keep track of the slots defined by each binary class and then include inherited slots in the **WITH-SLOTS** forms in the read-object and methods.

(setf (get 'foo 'slots) '(x y z))

You want this bookkeeping to happen as part of evaluating the define-binary-class of foo. However, it’s not clear where to put the expression. If you evaluate it when you compute the macro’s expansion, it’ll get evaluated when you compile the define-binary-class form but not if you later load a file that contains the resulting compiled code. On the other hand, if you include the expression in the expansion, then it won’t be evaluated during compilation, which means if you compile a file with several define-binary-class forms, none of the information about what classes define what slots will be available until the whole file is loaded, which is too late.

This is what the special operator **EVAL-WHEN** I discussed in Chapter 20 is for. By wrapping a form in an , you can control whether it’s evaluated at compile time, when the compiled code is loaded, or both. For cases like this where you want to squirrel away some information during the compilation of a macro form that you also want to be available after the compiled form is loaded, you should wrap it in an **EVAL-WHEN** like this:

and include the **EVAL-WHEN** in the expansion generated by the macro. Thus, you can save both the slots and the direct superclasses of a binary class by adding this form to the expansion generated by define-binary-class:

(eval-when (:compile-toplevel :load-toplevel :execute)
  (setf (get ',name 'slots) ',(mapcar #'first slots))

(defun direct-slots (name)
  (copy-list (get name 'slots)))

The next function returns the slots inherited from other binary classes.

Finally, you can define a function that returns a list containing the names of all directly defined and inherited slots.

(defun all-slots (name)
  (nconc (direct-slots name) (inherited-slots name)))

When you’re computing the expansion of a define-generic-binary-class form, you want to generate a **WITH-SLOTS** form that contains the names of all the slots defined in the new class and all its superclasses. However, you can’t use all-slots while you’re generating the expansion since the information won’t be available until after the expansion is compiled. Instead, you should use the following function, which takes the list of slot specifiers and superclasses passed to define-generic-binary-class and uses them to compute the list of all the new class’s slots:

  (nconc (mapcan #'all-slots superclasses) (mapcar #'first slots)))