**A library for secure smart contract development.** Build on a solid foundation of community-vetted code.
This repository hosts the Upgrade Safe variant of [OpenZeppelin Contracts], meant for use in upgradeable contracts. This variant is available as separate package called `@openzeppelin/contracts-upgrade-safe`.
* Implementations of standards like [ERC20](https://docs.openzeppelin.com/contracts/erc20) and [ERC721](https://docs.openzeppelin.com/contracts/erc721).
* Reusable [Solidity components](https://docs.openzeppelin.com/contracts/utilities) to build custom contracts and complex decentralized systems.
It follows all of the rules for xref:upgrades-plugins::writing-upgradeable.adoc[Writing Upgradeable Contracts]: constructors are replaced by initializer functions, state variables are initialized in initializer functions, and we additionally check for storage incompatibilities across minor versions.
* First-class integration with the [Gas Station Network](https://docs.openzeppelin.com/contracts/gsn) for systems with no gas fees!
*[Audited](https://github.com/OpenZeppelin/openzeppelin-contracts/tree/master/audit) by leading security firms (_last full audit on v2.0.0_).
OpenZeppelin Contracts features a [stable API](https://docs.openzeppelin.com/contracts/releases-stability#api-stability), which means your contracts won't break unexpectedly when upgrading to a newer minor version.
### Usage
### Usage
Once installed, you can use the contracts in the library by importing them:
The package replicates the structure of the main OpenZeppelin Contracts package, but every file and contract has the suffix `UpgradeSafe`.
Constructors are replaced by internal initializer functions following the naming convention `__{ContractName}_init`. Since these are internal, you must always define your own public initializer function and call the parent initializer of the contract you extend.
contract MyCollectible is ERC721 {
```diff
constructor() ERC721("MyCollectible", "MCO") public {
- constructor() ERC721("MyCollectible", "MCO") public {
}
+ function initialize() initializer public {
}
+ __ERC721_init("MyCollectible", "MCO");
}
```
```
> **Caution**
>
> Use with multiple inheritance requires special care. Initializer functions are not linearized by the compiler like constructors. Because of this, each `__{ContractName}_init` function embeds the linearized calls to all parent initializers. As a consequence, calling two of these `init` functions can potentially initialize the same contract twice.
>
> The function `__{ContractName}_init_unchained` found in every contract is the initializer function minus the calls to parent initializers, and can be used to avoid the double initialization problem, but doing this manually is not recommended. We hope to be able to implement safety checks for this in future versions of the Upgrades Plugins.
_If you're new to smart contract development, head to [Developing Smart Contracts](https://docs.openzeppelin.com/learn/developing-smart-contracts) to learn about creating a new project and compiling your contracts._
_If you're new to smart contract development, head to [Developing Smart Contracts](https://docs.openzeppelin.com/learn/developing-smart-contracts) to learn about creating a new project and compiling your contracts._
To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.
To keep your system secure, you should **always** use the installed code as-is, and neither copy-paste it from online sources, nor modify it yourself. The library is designed so that only the contracts and functions you use are deployed, so you don't need to worry about it needlessly increasing gas costs.
