Proxy contracts are being increasingly used as both as an upgradeability mechanism and a way to save gas when deploying many instances of a particular contract. This standard proposes a set of interfaces for proxies to signal how they work and what their main implementation is.
Using proxies that delegate their own logic to another contract is becoming an increasingly popular technique for both smart contract upgradeability and creating cheap clone contracts.
We don't believe there is value in standardizing any particular implementation of a DelegateProxy, given its simplicity, but we believe there is a lot of value in agreeing on an interface all proxies use that allows for a standard way to operate with proxies.
aragonOS: AppProxyUpgradeable, AppProxyPinned and KernelProxy
zeppelinOS: Proxy
implementation())The returned code address is the address the proxy would delegate calls to at that moment in time, for that message.
proxyType())Checking the proxy type is the way to check whether a contract is a proxy at all. When a contract fails to return to this method or it returns 0, it can be assumed that the contract is not a proxy.
It also allows for communicating a bit more of information about how the proxy operates. It is a pure function, therefore making it effectively constant as it cannot return a different value depending on state changes.
Forwarding proxy (id = 1): The proxy will always forward to the same code
address. The following invariant should always be true: once the proxy returns
a non-zero code address, that code address should never change.
Upgradeable proxy (id = 2): The proxy code address can be changed depending
on some arbitrary logic implemented either at the proxy level or in its forwarded
logic.
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