WTF Solidity: 25. Create2
Twitter: @0xAA_Science | @WTFAcademy_
Community: Discord｜Wechat｜Website wtf.academy
Codes and tutorials are open source on GitHub: github.com/AmazingAng/WTFSolidity
CREATE2 opcode helps us to predict the address of smart contract before it is deployed on Ethereum network, and Uniswap created Pair contract with CREATE2 instead of CREATE.
In this chapter, I will introduce the use of CREATE2.
How does CREATE calculate address
Smart contracts can be created by other contracts and regular accounts using the CREATE opcode.
In both cases, the address of new contract is calculated in the same way: the hash of creator's address (usually wallet address which will deploy or contract address) and the nonce(the total number of transactions sent from this address or, for contract account, the total number of contracts created. Every time a contract is created, the nonce will plus one).
new address = hash(creator's address, nonce)
creator's address won't change, but the nonce may change over time, so it's difficult to predict the address of contract created with CREATE.
How does CREATE2 calculate address
The purpose of CREATE2 is to make contract address independent of future events. No matter what happens on blockchain in the future, you can deploy the contract to a pre-calculated address.
The address of contract created with CREATE2 is determined by four parts:
0xFF: a constant to avoid conflict withCREATE- creator's address
- salt: a value given by creator
- The bytecode of contract to be deployed
new address = hash("0xFF", creator's address, salt, bytecode)
CREATE2 ensures that if creator deploys a given contract bytecode with CREATE2 and given salt, it will be stored at new address.
How to use CREATE2
CREATE2 is used in the same way as Create. It also new a new contract and passes in parameters which is needed for the new contract constructor, except with an extra salt parameter.
Contract x = new Contract{salt: _salt, value: _value}(params)
Contract is the name of contract to be created, x is the contract object (address), and _salt is the specified salt; If the constructor is payable, a number of(_value) ETH can be transferred to the contract at creation, and params is the parameter of new contract constructor.
Minimalist Uniswap2
Similar to the previous chapter, we use Create2 to implement a minimalist Uniswap.
Pair
contract Pair{
address public factory; // factory contract address
address public token0; // token1
address public token1; // token2
constructor() payable {
factory = msg.sender;
}
// called once by the factory at time of deployment
function initialize(address _token0, address _token1) external {
require(msg.sender == factory, 'UniswapV2: FORBIDDEN'); // sufficient check
token0 = _token0;
token1 = _token1;
}
}
Pair contract is simple and contains three state variables: factory, token0 and token1.
Constructor assigns the factory to factory contract address at deployment time. initialize function is called once by factory contract when the Pair contract is created, updating token0 and token1 to the addresses of two tokens in the token pair.
PairFactory2
contract PairFactory2{
mapping(address => mapping(address => address)) public getPair; // Find the Pair address by two token addresses
address[] public allPairs; // Save all Pair addresses
function createPair2(address tokenA, address tokenB) external returns (address pairAddr) {
require(tokenA != tokenB, 'IDENTICAL_ADDRESSES'); //Avoid conflicts when tokenA and tokenB are the same
// Calculate salt with tokenA and tokenB addresses
(address token0, address token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA); //Sort tokenA and tokenB by size
bytes32 salt = keccak256(abi.encodePacked(token0, token1));
//Deploy new contract with create2
Pair pair = new Pair{salt: salt}();
// call initialize function of the new contract
pair.initialize(tokenA, tokenB);
// Update address map
pairAddr = address(pair);
allPairs.push(pairAddr);
getPair[tokenA][tokenB] = pairAddr;
getPair[tokenB][tokenA] = pairAddr;
}
Factory contract(PairFactory2) has two state variables. getPair is a map of two token addresses to the token pair address. It is convenient to find the token pair address according to tokens. allPairs is an array of token pair address, storing all token pair addresses.
PairFactory2 contract has only one createPair2 function, which uses CREATE2 to create a new Pair contract based on the two token addresses tokenA and tokenB entered. Inside
Pair pair = new Pair{salt: salt}();
It's the above code that uses CREATE2 to create contract, which is very simple, and salt is the hash of token1 and token2.
bytes32 salt = keccak256(abi.encodePacked(token0, token1));
Calculate Pair address beforehand
// Calculate Pair contract address beforehand
function calculateAddr(address tokenA, address tokenB) public view returns(address predictedAddress){
require(tokenA != tokenB, 'IDENTICAL_ADDRESSES'); //Avoid conflicts when tokenA and tokenB are the same
// Calculate salt with tokenA and tokenB addresses
(address token0, address token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA); //Sort tokenA and tokenB by size
bytes32 salt = keccak256(abi.encodePacked(token0, token1));
// Calculate contract address
predictedAddress = address(uint160(uint(keccak256(abi.encodePacked(
bytes1(0xff),
address(this),
salt,
keccak256(type(Pair).creationCode)
)))));
}
We write a calculateAddr function to precompute the address of Pair that tokenA and tokenB will generate. With it, we can verify whether the address we calculated in advance is the same as actual address.
You can deploy PairFactory2 contract and call createPair2 with the following two addresses as parameters to see what is the address of token pair created and whether it is the same as the precomputed address.
WBNB address: 0x2c44b726ADF1963cA47Af88B284C06f30380fC78
PEOPLE address on BSC:
0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c
If there are parameters in deployment contract constructor
For example, when create2 contract:
Pair pair = new Pair{salt: salt}(address(this));
When calculating, you need to package parameters and bytecode together:
keccak256(type(Pair).creationCode)=> keccak256(abi.encodePacked(type(Pair).creationCode, abi.encode(address(this))))predictedAddress = address(uint160(uint(keccak256(abi.encodePacked(
bytes1(0xff),
address(this),
salt,
keccak256(abi.encodePacked(type(Pair).creationCode, abi.encode(address(this))))
)))));
Verify on remix
- First, the address hash of
WBNBandPEOPLEis used assaltto calculate the address ofPaircontract - Calling
PairFactory2.createPair2and the address ofWBNBandPEOPLEare passed in as parameters to get the address ofpaircontract created. - Compare contract address.
Application scenario of CREATE2
- The exchange reserves addresses for new users to create wallet contracts.
Factorycontract driven byCREATE2. The creation of trading pairs inUniswapV2is done by callingcreate2inFactory. The advantage is: It can get a certainpairaddress, so that the Router can calculatepairaddress through(tokenA, tokenB), no longer need to perform aFactory.getPair(tokenA, tokenB)cross-contract call.
Summary
In this chapter, we introduced the principle of CREATE2 opcode and how to use it. Besides, we used it to create a minimalist version of Uniswap and calculate token pair contract address in advance. CREATE2 helps us to determine contract address before deploying the contract, which is basis for some layer2 projects.