哪些语言允许更改对象的身份（不强制转换）？

发布于 2024-11-06 18:06:57 字数 252 浏览 5 评论 0 原文

在这篇文章中，一位勇敢的人希望（在 C++ 中）将 Base 类型的对象向下转换为派生 类型。假设 Derived 类型的属性不超过 Base，如果您嫉妒 Derived 类的额外方法，那么它可以有意义提供。

是否有编程语言允许这样的事情？

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雨巷深深 2024-11-13 18:06:57

实际上，这在 Common Lisp 以及移植了 CLOS（Common Lis 对象系统）的其他 Lisp 方言中是没有问题的。您使用 change-class 通用函数为此。

CLOS 使用多个调度方法，因此方法不依赖于类或对象，它只是在一组与其参数的类型（或标识）类似的函数中选择的函数。使用 change-class 时，您可以像创建新实例一样给出参数，并且已存储在对象中的数据将保留。下面是一个小会话，展示了它是如何工作的：

CL-USER> (defclass base ()
       ((name :initarg :name)))
#<STANDARD-CLASS BASE>
CL-USER> (defclass derived (base)
       ((age :initarg :age :initform 0)))
#<STANDARD-CLASS DERIVED>
CL-USER> (defvar foo (make-instance 'base :name "John Doe"))
FOO
CL-USER> (change-class foo 'derived :age 27)
#<DERIVED {100338F2D1}>
CL-USER> (with-slots (name age) foo
       (list name age))
("John Doe" 27)
CL-USER> (defvar bar (make-instance 'base :name "Baby Joe"))
BAR
CL-USER> (change-class bar 'derived)
#<DERIVED {10036CF6E1}>
CL-USER> (with-slots (name age) bar
       (list name age))
("Baby Joe" 0)
CL-USER>

如果这个默认行为还不够，您可以在 更新不同类的实例。

是的，有一些编程语言允许这样的事情！

Actually, this is something that is done without problem in Common Lisp, and in other Lisp dialects where CLOS (Common Lis Object System) was ported. You use the change-class generic function for that.

CLOS works with multiple dispatch methods, so a method is not tied to a class or object, it's just a function that is chosen in a group of similar functions WRT to the types (or identities) of its arguments. When using change-class, you can give arguments as if you were creating a new instance, and data already stored in the object will remain. Here is a little session that shows how it works:

CL-USER> (defclass base ()
       ((name :initarg :name)))
#<STANDARD-CLASS BASE>
CL-USER> (defclass derived (base)
       ((age :initarg :age :initform 0)))
#<STANDARD-CLASS DERIVED>
CL-USER> (defvar foo (make-instance 'base :name "John Doe"))
FOO
CL-USER> (change-class foo 'derived :age 27)
#<DERIVED {100338F2D1}>
CL-USER> (with-slots (name age) foo
       (list name age))
("John Doe" 27)
CL-USER> (defvar bar (make-instance 'base :name "Baby Joe"))
BAR
CL-USER> (change-class bar 'derived)
#<DERIVED {10036CF6E1}>
CL-USER> (with-slots (name age) bar
       (list name age))
("Baby Joe" 0)
CL-USER>

If this default behaviour is not enough, you may define a method on update-instance-for-different-class.

So yeah, there are programming languages that allow such a thing!

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