Roles for Owners

Ownership types were proposed to characterize the topology of objects on the heap. They notionally organize objects into boxes, and each box belongs to an object – its owner. In most ownership-based systems, the box protects the object from its environment in some way. Thus, the owner may play the owners-as-dominators role, whereby the owner restricts access to the objects in a box, (i.e. the “outside” may not access the “inside”), or, the owner may play the owners-asmodifiers role, whereby the owner restricts modification of the objects in a box (i.e. the “outside” may not modify the “inside”). We propose the dual protection, whereby the box protects the environment from the object. We suggest two further roles: in owners-as-restrictors, the owner restricts access from the object (i.e. the “inside” may not access the “outside”), and in owners-as-filters, the owner restricts the range of modifications from an object (i.e. the “inside” may not modify the “outside”). We explore the design space for possible exact meanings for the four roles. We define the meanings of these roles in terms of the guarantees they make about the heap and about executions. We sketch parts of a language which supports all four roles, and which allows any of the owner parameters to play any of these four roles. These roles may be enforced statically, dynamically, or via a combination of both.