Object ownership and containment

• Authors: David Gerard Clarke
• Venue: Object ownership and containment
• Year: 2003

Object-oriented programming relies on inter-object aliases to implement but it is when mutable state interacts with aliasing that problems arise. Through aliasing an object's state can be changed without the object being aware of the changes, potentially violating the object's invariants. This problem is fundamentally unresolvable. Many idioms such as the Observer design pattern rely on it. Hence aliasing cannot be eliminated from object-oriented programming, it can only be managed. Various proposals have appeared in the literature addressing the issue of alias management. The most promising are based on alias encapsulation, which limits access to objects to within certain well-defined boundaries. Our approach called ownership types falls into this category. An object can specify the objects it owns, called its representation, and which objects can access its representation. A type system protects the representation by enforcing a well-defined containment invariant. Our approach is a formal one. Ownership types are cast as a type system using an minor extension to Abadi and Cardelli's object calculus with subtyping. With this formalisation we prove the soundness of our ownership types system and demonstrate that well-typed programs satisfy the containment invariant. In addition, we also provide a firm grounding to enable ownership types to be safely added to an object-oriented programming language with inheritance, subtyping, and nested classes, as well as offering a sound basis for future work. Our type system can model aggregate objects with multiple interface objects sharing representation and friendly functions which access multiple objects' private representations, among other examples, thus overcoming weaknesses in existing alias management schemes.