Solidity - lesson 3 | HackIndia Skills

Solidity - lesson 3 | HackIndia Skills | HackIndia

Example

Owner and events

This pattern is small enough to understand. It sets an owner once, restricts admin actions, and emits an event when state changes.

Use custom errors instead of long revert strings when the error is part of your contract design.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.37;

contract CampusClub {
    address public immutable owner;
    string public notice;
    bool public paused;

    error NotOwner(address caller);
    error Paused();
    error EmptyNotice();

    event NoticeChanged(address indexed changedBy, string newNotice);
    event PausedChanged(address indexed changedBy, bool paused);

    modifier onlyOwner() {
        if (msg.sender != owner) revert NotOwner(msg.sender);
        _;
    }

    modifier whenNotPaused() {
        if (paused) revert Paused();
        _;
    }

    constructor(string memory firstNotice) {
        owner = msg.sender;
        notice = firstNotice;
    }

    function setNotice(string calldata newNotice) external onlyOwner whenNotPaused {
        if (bytes(newNotice).length == 0) revert EmptyNotice();
        notice = newNotice;
        emit NoticeChanged(msg.sender, newNotice);
    }

    function setPaused(bool value) external onlyOwner {
        paused = value;
        emit PausedChanged(msg.sender, value);
    }
}

Tip

Use OpenZeppelin when bigger

For a hackathon contract with simple ownership, writing the pattern yourself teaches you what is happening. For a larger repo, use audited building blocks instead of copy-pasting library code.

OpenZeppelin Contracts 5.x includes Ownable for onlyOwner, Ownable2Step for safer ownership transfer, and Pausable for emergency stops. Install the package instead of copying source files.

npm install @openzeppelin/contracts

Control state and access

One rule

State needs guards

Owner and events

Read the pattern

Do not use tx.origin

Use OpenZeppelin when bigger

Private is not secret

Do this now

Quick check