ERC-721: The Standard for Non-Fungible Tokens (NFTs)

Introduction

ERC-721 is the standard for non-fungible tokens (NFTs), meaning each token is unique and cannot be replaced (e.g., CryptoKitties, Bored Apes).

Architecture

An ERC-721 token smart contract includes:

• Token Ownership Mapping: Each token has a unique tokenId linked to an address.

• Metadata Storage: Stores information like name, image, and description.

• Transfer Functions: Allows ownership to be transferred.

Key Functions of ERC-721

Function	Description
balanceOf(address)	Returns the number of NFTs owned by an address
ownerOf(uint256 tokenId)	Returns the owner of a specific NFT
transferFrom(address, address, uint256)	Transfers ownership of an NFT
approve(address, uint256)	Approves another address to manage an NFT
safeTransferFrom(address, address, uint256)	Ensures safe NFT transfer

Workflow of ERC-721 Transactions

1. Minting: A unique tokenId is created and assigned to an owner.

2. Transfer: The NFT can be transferred using transferFrom().

3. Approval & Marketplace Integration:

   • The owner approves a marketplace contract to manage their NFT using approve().

   • The marketplace can then transfer the NFT on behalf of the owner.

Example: ERC-721 Smart Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract MyNFT is ERC721URIStorage, Ownable {
    uint256 private _nextTokenId;

    constructor() ERC721("MyNFT", "MNFT") {}

    function mintNFT(address recipient, string memory tokenURI) public onlyOwner {
        uint256 tokenId = _nextTokenId;
        _nextTokenId++;
        _mint(recipient, tokenId);
        _setTokenURI(tokenId, tokenURI);
    }
}

ERC-721 NFT Flow

[ User A ]  ---> (mint tokenId #1) --->  [ NFT Contract ]
[ User A ]  ---> (transfer tokenId #1) --->  [ User B ]
[ Marketplace ] ---> (approve & transfer NFT) ---> [ Buyer ]