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ooppointerslanguage-designvirtual-functions

Why aren't hot-swappable vtables a popular language feature?

In object-oriented programming, it's sometimes nice to be able to modify the behavior of an already-created object. Of course this can be done with relatively verbose techniques such as the strategy pattern. However, sometimes it would be nice to just completely change the type of the object by changing the vtable pointer after instantiation. This would be safe if, assuming you're switching from class A to class B:

  1. class B is a subclass of class A and does not add any new fields, or
  2. class B and class A have the same parent class. Neither do anything except override virtual functions from the parent class. (No new fields or virtual functions.)
  3. In either case, A and B must have the same invariants.

This is hackable in C++ and the D programming language, because pointers can be arbitrarily cast around, but it's so ugly and hard to follow that I'd be scared to do it in code that needs to be understood by anyone else. Why isn't a higher-level way to do this generally provided?

Solution

  • Because the mindset of most languages designers is too static.

    While such features are dangerous in the hand of programmers, they are necessary tools for library builders. For example, in Java one can create objects without calling a constructor (yes, you can!) but this power is only given to library designers. Still however, many features that library designers would kill for are alas not possible in Java. C# on the other hand is adding more and more dynamic features in each version. I am really looking forward to all the awesome libraries one can build using the upcoming DLR (dynamic language runtime).

    In some dynamic languages such as Smalltalk (and also as far as I know Perl and Python, but not Ruby) it is totally possible to change the class of an object. In Pharo Smalltalk you achieve this with

    object primitiveChangeClassTo: anotherObject

    which changes the class of object to that of anotherObject. Please note that this is not the same as object become: anotherObject which exchanges all pointers of both objects.