ERC-721 NFT standard in Blockchain / Solidity

Introduction

The ERC-721 standard defines a way to create unique digital items called NFTs on the Ethereum blockchain. It helps people prove ownership of one-of-a-kind things like art or collectibles.

You want to create a digital collectible that is unique, like a rare trading card.

You want to sell digital art where each piece is one of a kind.

You want to track ownership of virtual items in a game.

You want to prove ownership of a digital asset that cannot be copied.

You want to build a marketplace where users can buy and sell unique tokens.

Syntax

Blockchain / Solidity

interface IERC721 {
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    function balanceOf(address owner) external view returns (uint256 balance);
    function ownerOf(uint256 tokenId) external view returns (address owner);
    function safeTransferFrom(address from, address to, uint256 tokenId) external;
    function transferFrom(address from, address to, uint256 tokenId) external;
    function approve(address to, uint256 tokenId) external;
    function getApproved(uint256 tokenId) external view returns (address operator);
    function setApprovalForAll(address operator, bool _approved) external;
    function isApprovedForAll(address owner, address operator) external view returns (bool);
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
}

This is an interface in Solidity, the language for Ethereum smart contracts.

It defines functions and events that any NFT contract must implement to follow the ERC-721 standard.

Examples

Returns how many NFTs the owner has.

Blockchain / Solidity

function balanceOf(address owner) external view returns (uint256 balance);

Returns the owner address of a specific NFT by its unique ID.

Blockchain / Solidity

function ownerOf(uint256 tokenId) external view returns (address owner);

Transfers ownership of an NFT from one address to another.

Blockchain / Solidity

function transferFrom(address from, address to, uint256 tokenId) external;

Event emitted when an NFT is transferred. Helps track ownership changes.

Blockchain / Solidity

event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

Sample Program

This smart contract creates a simple NFT collection called 'MyUniqueArt'. Only the admin can create (mint) new unique tokens. Each token has a unique ID starting from 0.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract MyUniqueArt is ERC721 {
    uint256 public nextTokenId;
    address public admin;

    constructor() ERC721('MyUniqueArt', 'MUA') {
        admin = msg.sender;
    }

    function mint(address to) external {
        require(msg.sender == admin, 'only admin can mint');
        _safeMint(to, nextTokenId);
        nextTokenId++;
    }
}

Important Notes

ERC-721 tokens are unique and not interchangeable like regular cryptocurrencies.

Each NFT has a unique ID called tokenId to identify it.

Use libraries like OpenZeppelin to avoid mistakes and save time.

Summary

ERC-721 is the standard for unique digital tokens called NFTs on Ethereum.

It defines how to track ownership and transfer NFTs safely.

It is used for digital art, collectibles, and unique virtual items.

Practice

1. What is the main purpose of the ERC-721 standard in blockchain?

easy

A. To define a standard for unique non-fungible tokens (NFTs)

B. To create fungible tokens like cryptocurrencies

C. To manage smart contract deployment

D. To encrypt blockchain transactions

Solution

1. Step 1: Understand ERC-721's role

   ERC-721 is designed specifically for unique tokens, called NFTs, which are different from fungible tokens like cryptocurrencies.

2. Step 2: Compare with other token standards

   ERC-20 is for fungible tokens, while ERC-721 is for unique, non-fungible tokens.

3. Final Answer:

   To define a standard for unique non-fungible tokens (NFTs) -> Option A

4. Quick Check:

   ERC-721 = Unique NFTs [OK]

Hint: ERC-721 is for unique tokens, not fungible ones [OK]

Common Mistakes:

• Confusing ERC-721 with ERC-20 fungible tokens
• Thinking ERC-721 manages encryption
• Assuming ERC-721 handles contract deployment

2. Which of the following is the correct function signature to get the owner of an NFT in ERC-721?

easy

A. function getOwner(address token) public returns (uint256);

B. function ownerOf(uint256 tokenId) external view returns (address);

C. function owner(uint256 id) external returns (address);

D. function tokenOwner(uint256 tokenId) public view returns (string);

Solution

1. Step 1: Recall ERC-721 standard function names

   The standard defines ownerOf with a uint256 tokenId parameter returning an address.

2. Step 2: Check function visibility and return type

   The function is external view and returns an address, matching function ownerOf(uint256 tokenId) external view returns (address); exactly.

3. Final Answer:

   function ownerOf(uint256 tokenId) external view returns (address); -> Option B

4. Quick Check:

   ownerOf(tokenId) returns owner address [OK]

Hint: Look for 'ownerOf' with tokenId returning address [OK]

Common Mistakes:

• Using wrong function names like getOwner or tokenOwner
• Incorrect return types like string instead of address
• Missing view or external keywords

3. Given the following Solidity snippet, what will be the output of ownerOf(1) if token 1 is owned by address 0xABC...?

contract MyNFT is ERC721 {
    mapping(uint256 => address) private _owners;
    
    function ownerOf(uint256 tokenId) public view override returns (address) {
        return _owners[tokenId];
    }

    function mint(address to, uint256 tokenId) public {
        _owners[tokenId] = to;
    }
}

medium

A. 0x0000000000000000000000000000000000000000

B. Compilation error

C. 0xABC...

D. Function does not return anything

Solution

1. Step 1: Understand the mint function effect

   The mint function sets the owner of tokenId to the given address in the _owners mapping.

2. Step 2: Check ownerOf return value

   The ownerOf function returns the address stored in _owners[tokenId]. If token 1 was minted to 0xABC..., it returns that address.

3. Final Answer:

   0xABC... -> Option C

4. Quick Check:

   ownerOf(1) = minted owner address [OK]

Hint: ownerOf returns stored owner address from mapping [OK]

Common Mistakes:

• Assuming default zero address if not minted
• Thinking function has no return
• Confusing compilation errors with runtime behavior

4. Identify the error in this ERC-721 transfer function snippet:

function transferFrom(address from, address to, uint256 tokenId) public override {
    require(ownerOf(tokenId) == msg.sender, "Not owner");
    _owners[tokenId] = to;
}

medium

A. The require should check if msg.sender is approved or owner, not just owner

B. The function should not override transferFrom

C. The _owners mapping should not be updated

D. The function is missing a return statement

Solution

1. Step 1: Check transferFrom permission logic

   ERC-721 requires that the caller is either the owner or approved to transfer the token, not just the owner.

2. Step 2: Identify missing approval check

   The code only checks if msg.sender is the owner, missing approval checks, which can block valid transfers.

3. Final Answer:

   The require should check if msg.sender is approved or owner, not just owner -> Option A

4. Quick Check:

   transferFrom needs owner or approved check [OK]

Hint: Check both owner and approved in transferFrom require [OK]

Common Mistakes:

• Only checking owner without approval
• Forgetting to override transferFrom
• Not updating ownership mapping

5. You want to create an NFT contract that allows only the original minter to burn their token. Which ERC-721 function should you override and what check should you add?

hard

A. Override mint and check if msg.sender is approved

B. Override transferFrom and check if msg.sender is the owner

C. Override approve and check if msg.sender is the contract owner

D. Override _burn and check if msg.sender is the original minter

Solution

1. Step 1: Identify burn function in ERC-721

   The _burn internal function handles token destruction in ERC-721 contracts.

2. Step 2: Add ownership check for burning

   To restrict burning to original minters, override _burn and add a check that msg.sender matches the original minter address stored during minting.

3. Final Answer:

   Override _burn and check if msg.sender is the original minter -> Option D

4. Quick Check:

   Burn control = override _burn + minter check [OK]

Hint: Control burn by overriding _burn with minter check [OK]

Common Mistakes:

• Trying to control burn in transferFrom or approve
• Not storing original minter address
• Checking approval instead of minter for burn