Ethereum-Solidity Quiz Q14: Why constructors can't be used in upgradeable contracts?

#web3 #ethereum #solidity #blockchain

In Solidity, code that is inside a constructor or part of a global variable declaration is not part of a deployed contract’s runtime bytecode. This code is executed only once, when the contract instance is deployed. As a consequence of this, the code within a logic contract’s constructor will never be executed in the context of the proxy’s state. To rephrase, proxies are completely oblivious to the storage changes that are performed by the constructor.

The problem is easily solved though. Logic contracts should move the code within the constructor to a regular 'initializer' function, and have this function be called whenever the proxy links to this logic contract. Special care needs to be taken with this initializer function so that it can only be called once, which is one of the properties of constructors in general programming.

This is why when we create a proxy using OpenZeppelin Upgrades, you can provide the name of the initializer function and pass parameters.

To ensure that the initialize function can only be called once, a simple modifier is used. OpenZeppelin Upgrades provides this functionality via a contract that can be extended:

// contracts/MyContract.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";

contract MyContract is Initializable {
    function initialize(
        address arg1,
        uint256 arg2,
        bytes memory arg3
    ) public payable initializer {
        // "constructor" code...

    }
}