Immutable and Constant

• Using the constant and the immutable keywords for variables that do not change helps to save on gas used.

• constant and immutable variables do not occupy a storage slot when compiled.

• They are saved inside the contract byte code.

What is a constant variable?

A constant variable in Solidity is a variable whose value cannot be changed once set. This means that a constant variable can only be assigned a value once, and it cannot be reassigned or modified thereafter.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract Constants {
    address public constant myAddress = 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc;
    uint public constant myUint = 143;
}

• assignment has to be done at declaration

What is an immutable variable?

An immutable variable in Solidity refers to a variable whose reference cannot be changed. In other words, an immutable variable is a variable that points to an object, and the object’s value cannot be changed, but the variable can be reassigned to point to a new object.

For example, if you have an immutable variable named “y” that points to a string “Hello”, you cannot change the string to “Goodbye”, but you can reassign “y” to point to a new string “Hola”.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract Immutable {
    address public immutable myAddress;
    uint public immutable myUint;

    constructor(uint _myUint) {
        myAddress = msg.sender;
        myUint = _myUint;
    }
}

• Values of immutable variables can be set inside the constructor but cannot be modified afterward.

Constants are typically used when you want to ensure that a value cannot be changed, while immutables are used when you want to ensure that a reference cannot be changed.