Editing Fragile base class (section)

{{Short description|Object-oriented programming system problem}}{{More sources|date=November 2024}}{{About|the architectural problem|the language implementation problem|Fragile binary interface problem}}The '''fragile base class problem''' is a fundamental architectural problem of [[object-oriented programming]] systems where base classes ([[superclass (computer science)|superclass]]es) are considered "fragile" because seemingly safe modifications to a base class, when inherited by the [[subclass (computer science)|derived class]]es, may cause the derived classes to malfunction. The programmer cannot determine whether a base class change is safe simply by examining in isolation the methods of the base class.

One possible solution is to make instance variables private to their defining class and force subclasses to use accessors to modify superclass states. A language could also make it so that subclasses can control which inherited methods are exposed publicly. These changes prevent subclasses from relying on implementation details of superclasses and allow subclasses to expose only those superclass methods that are applicable to themselves.

An alternative solution is to have an [[Interface (object-oriented programming)|interface]] instead of superclass.

The fragile base class problem has been blamed on [[open recursion]] (dynamic dispatch of methods on <code>[[this (computer programming)|this]]</code>), with the suggestion that invoking methods on <code>this</code> default to closed recursion (static dispatch, early binding) rather than open recursion (dynamic dispatch, late binding), only using open recursion when it is specifically requested; external calls (not using <code>this</code>) would be dynamically dispatched as usual.<ref>"[https://www.cs.cmu.edu/~aldrich/papers/selective-open-recursion.pdf Selective Open Recursion: A Solution to the Fragile Base Class Problem]", Jonathan Aldrich</ref><ref>"[http://lambda-the-ultimate.org/classic/message12271.html Selective Open Recursion: A Solution to the Fragile Base Class Problem]", ''[http://lambda-the-ultimate.org/ Lambda the Ultimate]''</ref>