Delegatecall: The Detailed and Animated Guide

This article explains how delegatecall works in detail. The Ethereum Virtual Machine (EVM) offers four opcodes for making calls between contracts:

• CALL (F1)
• CALLCODE (F2)
• STATICCALL (FA)
• and DELEGATECALL (F4).

Notably, the CALLCODE opcode has been deprecated since Solidity v5, being replaced by DELEGATECALL. These opcodes have a direct implementation in Solidity and can be executed as methods of variables of type address.

To gain a better understanding of how delegatecall works, let’s first review the functionality of the CALL opcode.

CALL

To demonstrate call consider the following contract:

contract Called {
  uint public number;

function increment() public {
    number++;
  }
}

The most direct way to execute the increment() function from another contract is by utilizing the Called contract interface. In this recipe, we can execute the function with a statement as simple as called.increment() where called is the address of Called. But calling increment() can also be achieved using a low-level call as show in the contract below:

contract Caller {

address constant public calledAddress = 0xd9145CCE52D386f254917e481eB44e9943F39138; // Called's address

function callIncrement() public {
        calledAddress.call(abi.encodeWithSignature("increment()"));
    }
}