Where is changing the class of an object at runtime allowed

Do you know programming languages where changing the class of an object at runtime is allowed (supported)?

Please give a short example regarding the syntax. Give a use case, if you know any. Examples involving duck typing are welcome as well, so do not shy away from mentioning these languages.

Update: I fi开发者_开发技巧gured out that Smalltalk has changeClassTo and become. CLOS can do change-class. I found a paper suggesting to use these mechanisms to implement 'husk objects' that are referenced at runtime, but only constructed from some persistence when actually accessed, providing some nifty lazy loading of related objects.

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I assume, you mean the following:

You have an object of class A. But you would like to treat it as an object of class B.

There are some constructions possible:

1. If B is a subclass of A you can cast the object to B (but it should be created as B else you have unexpected (and hopefully unwanted) results).

2. In some languages you can cast anything to anything. If you know what you are doing, this is great, else prepare for several holes in your foot.

You mention ducktyping. I have no practical experience with it. But As far as I know, duck typing is something like this: "I need an object that support methods X, Y and Z." In that case you don't care about the class. You just want it to quack, swim and walk at your command.

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Give a usecase

??? I'd expect you to ask for a solution on a specific use case. Changing type of an object? I think "No."

But if you like to change part of an objects capabilities or behaviours have a look at loosely coupling! For example your class holds a member of type File_Saver. There's a public setter accepting any instance of File_Saver and you can inject File_Saver_XML, File_Saver_PDF, ...

It's no common way, but any processing inside a class can be done by 1-n loosely coupled handlers, which you can exchange from outside.

Melt down to your question: You need a wrapper + a setter. :-)

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Coming back to the case after some time, I've come to the conclusion that you want duck typing if you feel the need of changing an objects class.