Cooperative Permissions for Reasoning About Aliased Objects

Maintaining object invariants is notoriously difficult when objects involved in invariants are aliased. Existing approaches achieve soundness in reasoning about object invariants by imposing structural constraints on object graphs, excluding many useful programs from being verified. This paper proposes a novel abstraction, cooperative permissions, for sound reasoning about aliased objects. Cooperative permissions describe not only what aliases are allowed to do, but also restrict possible effects to the referenced object through other aliases. Therefore, cooperative permissions are a local approach to keeping assumptions that aliases make about the referenced object globally consistent, allowing sound modular reasoning without imposing structural constraints. The paper illustrates with examples how cooperative permissions can be used to specify object behavior and gives an intuition of how such specifications could be verified.

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