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copied!<p>Everybody seems to be assuming a C++/Java-like object system, possibly because of the syntax used in the question, but it is quite possible to change the class of an instance at runtime in other languages.</p> <p>Lisp's CLOS allows changing the class of an instance at any time, and it's a well-defined and efficient transformation. (The terminology and structure is slightly different: methods don't "belong" to classes in CLOS.)</p> <p>I've never heard a name for this specific type of transformation, though. The function which does this is simply called <code>change-class</code>.</p> <p>Richard Gabriel <a href="http://www.dreamsongs.com/Files/clos-cacm.pdf" rel="nofollow noreferrer">seems to call it</a> the "change-class protocol", after Kiczales' <a href="http://www.cliki.net/AMOP" rel="nofollow noreferrer">AMOP</a>, which formalized as "protocols" many of the internals of CLOS for metaprogramming.</p> <p>People wonder why you'd want to do this; I see two big advantages over simply creating a new instance:</p> <ul> <li>faster: changing class can be as simple as updating a pointer, and updating any slots that differ; if the classes are very similar, this can be done with no new memory allocations</li> <li>simpler: if a dozen places already have a reference to the old object, creating a new instance won't change what they point to; if you need to update each one yourself, that could add a lot of complexity for what should be a simple operation (2 words, in Lisp)</li> </ul> <p>That's not to say it's always the right answer, but it's nice to have the ability to do this when you want it. "Change an instance's class" and "make a new instance that's similar to that one" are very different operations, and I like being able to say exactly what I mean.</p>

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