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Solidity API ​

Initializable ​

_This is a base contract to aid in writing upgradeable contracts, or any kind of contract that will be deployed behind a proxy. Since proxied contracts do not make use of a constructor, it's common to move constructor logic to an external initializer function, usually called initialize. It then becomes necessary to protect this initializer function so it can only be called once. The {initializer} modifier provided by this contract will have this effect.

The initialization functions use a version number. Once a version number is used, it is consumed and cannot be reused. This mechanism prevents re-execution of each "step" but allows the creation of new initialization steps in case an upgrade adds a module that needs to be initialized.

For example:

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solidity
contract MyToken is ERC20Upgradeable {
    function initialize() initializer public {
        __ERC20_init("MyToken", "MTK");
    }
}

contract MyTokenV2 is MyToken, ERC20PermitUpgradeable {
    function initializeV2() reinitializer(2) public {
        __ERC20Permit_init("MyToken");
    }
}

TIP: To avoid leaving the proxy in an uninitialized state, the initializer function should be called as early as possible by providing the encoded function call as the _data argument to {ERC1967Proxy-constructor}.

CAUTION: When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or to ensure that all initializers are idempotent. This is not verified automatically as constructors are by Solidity.

[CAUTION] ​

Avoid leaving a contract uninitialized.

An uninitialized contract can be taken over by an attacker. This applies to both a proxy and its implementation contract, which may impact the proxy. To prevent the implementation contract from being used, you should invoke the {_disableInitializers} function in the constructor to automatically lock it when it is deployed:

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/// @custom:oz-upgrades-unsafe-allow constructor
constructor() {
    _disableInitializers();
}

====_

InitializableStorage ​

_Storage of the initializable contract.

It's implemented on a custom ERC-7201 namespace to reduce the risk of storage collisions when using with upgradeable contracts._

solidity
struct InitializableStorage {
  uint64 _initialized;
  bool _initializing;
}

InvalidInitialization ​

solidity
error InvalidInitialization()

The contract is already initialized.

NotInitializing ​

solidity
error NotInitializing()

The contract is not initializing.

Initialized ​

solidity
event Initialized(uint64 version)

Triggered when the contract has been initialized or reinitialized.

initializer ​

solidity
modifier initializer()

_A modifier that defines a protected initializer function that can be invoked at most once. In its scope, onlyInitializing functions can be used to initialize parent contracts.

Similar to reinitializer(1), except that in the context of a constructor an initializer may be invoked any number of times. This behavior in the constructor can be useful during testing and is not expected to be used in production.

Emits an {Initialized} event._

reinitializer ​

solidity
modifier reinitializer(uint64 version)

_A modifier that defines a protected reinitializer function that can be invoked at most once, and only if the contract hasn't been initialized to a greater version before. In its scope, onlyInitializing functions can be used to initialize parent contracts.

A reinitializer may be used after the original initialization step. This is essential to configure modules that are added through upgrades and that require initialization.

When version is 1, this modifier is similar to initializer, except that functions marked with reinitializer cannot be nested. If one is invoked in the context of another, execution will revert.

Note that versions can jump in increments greater than 1; this implies that if multiple reinitializers coexist in a contract, executing them in the right order is up to the developer or operator.

WARNING: Setting the version to 2**64 - 1 will prevent any future reinitialization.

Emits an {Initialized} event._

onlyInitializing ​

solidity
modifier onlyInitializing()

Modifier to protect an initialization function so that it can only be invoked by functions with the {initializer} and {reinitializer} modifiers, directly or indirectly.

_checkInitializing ​

solidity
function _checkInitializing() internal view virtual

Reverts if the contract is not in an initializing state. See {onlyInitializing}.

_disableInitializers ​

solidity
function _disableInitializers() internal virtual

_Locks the contract, preventing any future reinitialization. This cannot be part of an initializer call. Calling this in the constructor of a contract will prevent that contract from being initialized or reinitialized to any version. It is recommended to use this to lock implementation contracts that are designed to be called through proxies.

Emits an {Initialized} event the first time it is successfully executed._

_getInitializedVersion ​

solidity
function _getInitializedVersion() internal view returns (uint64)

Returns the highest version that has been initialized. See {reinitializer}.

_isInitializing ​

solidity
function _isInitializing() internal view returns (bool)

Returns true if the contract is currently initializing. See {onlyInitializing}.