Variables, Consts & Immutable

Variables

There are 3 types of variables in Solidity:

• local

  • Declared inside a function.

  • Not stored on the blockchain.

• state

  • Declared outside a function.

  • Stored on the blockchain.

• global

  • Provides information about the blockchain.

Example - Variables

• https://solidity-by-example.org/variables/

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

contract Variables {
    // State variables are stored on the blockchain.
    string public text = "Hello";
    uint public num = 123;

    function doSomething() public {
        // Local variables are not saved to the blockchain.
        uint i = 456;

        // Here are some global variables
        uint timestamp = block.timestamp; // Current block timestamp
        address sender = msg.sender; // address of the caller
    }
}

Constants

• Constants are variables that cannot be modified

• Their value is hard coded into the bytecode of the contract

• Using constants can save gas cost

Example - Constants

• https://solidity-by-example.org/constants/

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

contract Constants {
    // coding convention to uppercase constant variables
    address public constant MY_ADDRESS = 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc;
    uint public constant MY_UINT = 123;
}

Immutable

• Immutable variables are like constants

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

Example - Immutable

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

contract Immutable {
    // coding convention to uppercase constant variables
    address public immutable MY_ADDRESS;
    uint public immutable MY_UINT;

    constructor(uint _myUint) {
        MY_ADDRESS = msg.sender;
        MY_UINT = _myUint;
    }
}