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Asked: 2026-05-29T10:44:06+00:00

I got a list of Objects of class-type B (B inherits from A). How

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I got a list of Objects of class-type B (B inherits from A).
How can I downcast all the B-objects to A-objects without using a function to pull the needed information and create a new instance?

(defclass A () ( (varI :initarg :varI :accessor varI) ) )

(defclass B (A) ( (varII :initarg :varII :accessor varII) ) )

(defun generate-list-of-type-B-objects () (....does...some..stuff....))

(defvar *listoftypeA* (generate-list-of-type-B-objects) )

(I know this example is easy to rewrite, as I could make the method generate a list of type A objects, but the function is used somewhere else where type-B objects are needed, and I do not want to duplicate code)

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1 Answer

  1. Editorial Team

    If you absolutely must convert your instances of class B into instances of class A, you can do a one-way conversion of them with the use of CHANGE-CLASS. This is a non-reversible change of each instance.

    Since I am not 100% sure what you actually want, the best suggestion I can give is “have you tried leaving them as class B and see if it works?”

    Chances are that it will just work, unless you have methods of a generic function somewhere that treat instances of class B differently than instances of class A. If it’s only about dropping the space that the extra slot would take, have you measured that it’s actually worth it (it probably isn’t, unless you have several thousands of instances of class B, where you could get by with instances of class A).

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