How to Write an ERC721 Token
ERC-721 introduced the standard for NFTs by making each Token unique and capable of having a different value than other Tokens from the same Smart Contract, maybe due to age, rarity or other factors. All NFTs have a
uint256 variable called tokenId, so for any ERC-721 Contract, the pair contract address, uint256 tokenId must be globally unique. That said, a dapp can have a "converter" that uses the tokenId as input and outputs an image of something cool like zombies, skills, or amazing kitties!An ERC-721 compatible token must implement the following methods and events.
function balanceOf(address _owner) external view returns (uint256);
function ownerOf(uint256 _tokenId) external view returns (address);
function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
function approve(address _approved, uint256 _tokenId) external payable;
function setApprovalForAll(address _operator, bool _approved) external;
function getApproved(uint256 _tokenId) external view returns (address);
function isApprovedForAll(address _owner, address _operator) external view returns (bool);
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);
We recommend using the OpenZeppelin ERC-721 implementation in your project. The source code can be found at ERC721.sol.
1. Install OpenZeppelin smart contracts in your project
npm install @openzeppelin/contracts2. Once installed, you can use the contracts in the library by importing them
// contracts/GameItem.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/utils/Counters.sol";
contract GameItem is ERC721URIStorage {
using Counters for Counters.Counter;
Counters.Counter private _tokenIds;
constructor() ERC721("GameItem", "ITM") {}
function awardItem(address player, string memory tokenURI)
public
returns (uint256)
{
uint256 newItemId = _tokenIds.current();
_mint(player, newItemId);
_setTokenURI(newItemId, tokenURI);
_tokenIds.increment();
return newItemId;
}
}