Solidity

What are Contracts?

All Solidity code is written inside contracts. A contract is a collection of code (its functions) and data (its state) that lives at a specific address on the Ethereum blockchain. It is the fundamental building block of every Ethereum application.

POP vs OOP vs BOP

This course compares three programming styles. “BOP – Block Oriented Programming” is the name used in this course to describe contract-based programming on a blockchain.

What Can a Contract Contain?

Kinds of Contract-like Units

• contract – a normal deployable contract.
• abstract contract – has at least one function without implementation (or is marked abstract); it cannot be deployed and is used as a base for inheritance (Chapter 18).
• interface – only function declarations, no implementation or state (Chapter 19).
• library – reusable code with no state of its own, e.g. maths helpers; its functions are called by contracts.

Example : One Contract Creating Another

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract Wallet {
    address public owner;
    string public label;

    constructor(address _owner, string memory _label) {
        owner = _owner;
        label = _label;
    }
}

contract WalletFactory {
    Wallet[] public wallets;                       // contract type as a variable type

    function createWallet(string calldata label) external returns (address) {
        Wallet w = new Wallet(msg.sender, label);  // deploys a new contract
        wallets.push(w);
        return address(w);                         // address of the new contract
    }

    function ownerOf(uint index) external view returns (address) {
        return wallets[index].owner();             // call a getter of another contract
    }

    function totalWallets() external view returns (uint) {
        return wallets.length;
    }
}

Key Takeaways

The PDF E-Notes are the earlier printable version of this chapter. The notes on this page are the latest, revised version.

Video : Contracts in Solidity

Resource File

• Chapter 16 : Solidity File

Question / Answer (15 questions)

MCQ Test : Check Your Understanding (5 questions)

Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.

Bibliography/References

The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.

1. Solidity Team. (n.d.). Contracts (creating contracts, visibility and getters, function modifiers, constant and immutable, functions, events, custom errors, inheritance, abstract contracts, interfaces, libraries). Solidity Documentation. https://docs.soliditylang.org/en/latest/contracts.html
2. Solidity Team. (n.d.). Structure of a Contract. Solidity Documentation. https://docs.soliditylang.org/en/latest/structure-of-a-contract.html
3. Solidity Team. (n.d.). Layout of a Solidity Source File (SPDX, pragmas, imports, comments). Solidity Documentation. https://docs.soliditylang.org/en/latest/layout-of-source-files.html
4. The Linux Foundation. (n.d.). About SPDX (ISO/IEC 5962:2021). https://spdx.dev/about/
5. OpenZeppelin. (n.d.). OpenZeppelin Contracts Documentation. https://docs.openzeppelin.com/contracts/
6. Modi, R. (2018). Solidity Programming Essentials: A beginner’s guide to build smart contracts for Ethereum and blockchain. Packt Publishing. ISBN 978-1-78883-138-3.