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Trustworthy Proxies: Virtualizing Objects with Invariants

Tom Van Cutsem
Mark S. Miller
ECOOP 2013


Proxies are a common technique to virtualize objects in object-oriented languages. A proxy is a placeholder object that emulates or wraps another target object. Both the proxy's representation and behavior may differ substantially from that of its target object. In many object-oriented languages, objects may have language-enforced invariants associated with them. For instance, an object may declare immutable fields, which are guaranteed to point to the same value throughout the execution of the program. Clients of an object can blindly rely on these invariants, as they are enforced by the language. In a language with both proxies and objects with invariants, these features interact. Can a proxy emulate or replace a target object purporting to uphold such invariants? If yes, does the client of the proxy need to trust the proxy to uphold these invariants, or are they still enforced by the language? This paper sheds light on these questions in the context of a Javascript-like language, and describes the design of a Proxy API that allows proxies to emulate objects with invariants, yet have these invariants continue to be language-enforced. This design forms the basis of proxies in ECMAScript 6.