Delegatecall is a function in Solidity, that lets you execute another contract’s function while using the original contract’s storage, msg.value and msg.sender.

I’ll explain it with an example.

Below we have two contracts. They both have the same layout in storage (a string and an address).

The only place that greeting or greetingLastUpdatedBy is set is in contract B.

But we want to use that function (setGreeting) inside contract A.

• First we would deploy contract B and get its address.
• Then we deploy contract A
• Then call setOtherContract() on contract A, so that contract A knows the address of the deployed contract B.
• Then call sayHello() on contract A. This will delegate a call to otherContract, calling the setGreeting function but it will use the contract A’s storage.
• If you look at the data in contract A, it will be unchanged
• But look at the data in contract B and you will see greeting and greetingLastUpdatedBy was updated.

pragma solidity ^0.8.4;

contract B {
    string public greeting;
    address public greetingLastUpdatedBy;

    function setGreeting(string _greeting) public {
        greeting = _greeting;
        greetingLastUpdatedBy = msg.sender;
    }
}

contract A {
    string public greeting;
    address public greetingLastUpdatedBy;
    address public otherContract;
    
    function setOtherContract(address _addr) public {
        otherContract = _addr;
    }

    function sayHello() public {
        (bool success, bytes memory data) = otherContract.delegatecall(
            abi.encodeWithSignature("setGreeting(string)", "Hello, world")
        );
    }
}

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