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Review structure of Proxies API docs (#2645)

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Francisco Giordano
2021-04-27 09:29:13 -03:00
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docs/modules/ROOT/pages/upgradeable.adoc
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= Using with Upgrades
If your contract is going to be deployed with upgradeability, such as using the xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins], you will need to use the Upgrade Safe variant of OpenZeppelin Contracts.
If your contract is going to be deployed with upgradeability, such as using the xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins], you will need to use the Upgradeable variant of OpenZeppelin Contracts.
This variant is available as a separate package called `@openzeppelin/contracts-upgradeable`, which is hosted in the repository https://github.com/OpenZeppelin/openzeppelin-contracts-upgradeable[OpenZeppelin/openzeppelin-contracts-upgradeable].
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=== Storage Gaps
You may notice that every contract includes a state variable named `+__gap+`. This is empty reserved space in storage that is put in place in Upgrade Safe contracts. It allows us to freely add new state variables in the future without compromising the storage compatibility with existing deployments.
You may notice that every contract includes a state variable named `+__gap+`. This is empty reserved space in storage that is put in place in Upgradeable contracts. It allows us to freely add new state variables in the future without compromising the storage compatibility with existing deployments.
It isn't safe to simply add a state variable because it "shifts down" all of the state variables below in the inheritance chain. This makes the storage layouts incompatible, as explained in xref:upgrades-plugins::writing-upgradeable.adoc#modifying-your-contracts[Writing Upgradeable Contracts]. The size of the `+__gap+` array is calculated so that the amount of storage used by a contract always adds up to the same number (in this case 50 storage slots).