How to build and use ERC-721 tokens in 2021
An intro for devs to the uniquely identifying token standard and its future
The ERC-721 standard has been around for a while now. Originally made popular by blockchain games, it's more and more used for other applications like Defi.
But what exactly is it?
A non-fungible token (NFT) is a uniquely identifying token. The word non-fungible implies you cannot just replace one NFT with another one. Every NFT is unique and different. In contrast to any fungible token like Bitcoin or ERC-20 where it doesn't matter which specific coin/token you receive, they all have the same value.
On Ethereum Cryptokitties helped to make this into the now popular 721 standard.
The Features of ERC-721
Let's explore the exact definition of the standard, all features and functions that are part of it.
1. The NFT implementation
At the core we have a mapping from uint256 => address. This is the reverse of a fungible token (e.g. ERC-20) mapping which is from address => uint256.
Why this reverse mapping?
In the fungible tokens we map from owner to balance. While in 721 we actually also do this in a second mapping (see balanceOf below), we more importantly need a way to uniquely identify tokens. By mapping from uint256 to address, we can give every minted token an id and map this id to exactly one owner. When creating a new NFT token, you create a new id (most commonly you just start at id 1 and count + 1 any following id) and set the owner as ownerMapping[id] = receiverAddress.
Let's look at the actual 721 functions:
A. Getters
function ownerOf(uint256 tokenId) external view returns (address);Returns the owner of the NFT token with the given id. In the 721 implementation we would just return ownerMapping[id] here.
function balanceOf(address owner) external view returns (uint256);Returns the balance of an owner, meaning the total amount of how many NFT tokens this address owns. In our implementation we will need to keep track of this when minting or transferring an NFT.
B. Transfer
function safeTransferFrom(address from, address to, uint256 tokenId) external payable;The transfer function comes in three forms. This one here is probably the most commonly used one. It will transfer the token with the given id to the receiving address. It will only work if the from address is the owner or approved (see below for approvals) and if the receiver is either not a smart contract or a smart contract that implements the 721 receiver interface (see below for ERC-165 and how to receive tokens). Otherwise the call will revert.
The other two variants for the transfer functions are simply one with an additional bytes data field to pass to the receiver hook and another non-safe transfer function that doesn't invoke the transfer hook.
C. Approvals
function approve(address approveTo, uint256 tokenId) external payable;
function getApproved(uint256 tokenId) external view returns (address);
function setApprovalForAll(address operator, bool isApproved) external;
function isApprovedForAll(address owner, address operator) external view returns (bool);
The approval functions allow someone to approve another address for either one specific token or all tokens. Think of the approve for all functions as something where you would approve a trusted smart contract, so it cannot just steal all your tokens. Once you approve another address, it has full transfer control over it.
2. Metadata (optional)
The metadata extension is optional for ERC-721 smart contracts. It includes a name and symbol just like they exist in many other token standards including ERC-20. Further a tokenURI can be defined for a token. Each unique token should have its own URI.
interface ERC721Metadata {
function name() external view returns (string name);
function symbol() external view returns (string symbol);
function tokenURI(uint256 tokenId) external view returns (string);
}This tokenURI links to a metadata file. In the case of a game, the file could just be hosted on the servers of the game provider. In the spirit of decentralization, in many cases IPFS is used to store this file. Just make sure to use IPFS pinning not to loose the file. The metadata file has a defined name, description and image. It might look like this:
{
"name": "Buzz",
"description": "Paper collage, using salvaged and original watercolour papers",
"image": "https://ipfs.infura.io/ipfs/QmWc6YHE815F8kExchG9kd2uSsv7ZF1iQNn23bt5iKC6K3/image"
}3. Enumeration (optional)
Another additional optional interface is the enumeration. It contains functions for counting and receiving tokens by the index.
interface ERC721Enumerable {
function totalSupply() external view returns (uint256);
function tokenByIndex(uint256 _index) external view returns (uint256);
function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256);
}With the totalSupply function you can determine how many NFT's in total exist currently, excluding the burnt ones of course. The other two functions will return the n'th token from the list of all tokens (tokenByIndex) or from the list of the tokens of that owner (tokenOfOwnerByIndex).
You might have noticed, if you never burn tokens and count token ids from 1 incrementally, the first two functions would just reflect the token ids.
4. Receiver hook and ERC-165 support
If you don't know what EIP-165 is yet, see my tutorial here. In short it's a way for smart contracts to define what interfaces they support. So for example, does the smart contract support receiving ERC-721 tokens? If it does, the respective supportsInterface function must exist and return true for that contract.
In the ERC-721 a smart contract receiver must implement the onERC721Received function. You could use the onERC721Received function to implement some further receiving logic or just confirm the receiver supports the 721. But be aware that in the case of the non-safe transfer function, the hook is not called! In the hook you always need to return the magic value as confirmation.
How can I deploy a 721 NFT?
Now let's see how you would implement a 721 token contract. We will be using the Openzeppelin contracts to help us.
This is a basic NFT example contract which allows the owner to mint new tokens. The syntax for the imports assumes using Remix, adjust it in case you installed the Openzeppelin contracts via npm.
// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.4/contracts/token/ERC721/ERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.4/contracts/utils/Counters.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.4/contracts/access/Ownable.sol";
contract NftExample is ERC721, Ownable {
using Counters for Counters.Counter;
Counters.Counter private _tokenIds;
constructor() ERC721("NFT-Example", "NEX") {}
function mintNft(address receiver, string memory tokenURI) external onlyOwner returns (uint256) {
_tokenIds.increment();
uint256 newNftTokenId = _tokenIds.current();
_mint(receiver, newNftTokenId);
_setTokenURI(newNftTokenId, tokenURI);
return newNftTokenId;
}
}Consider using some other minting mechanism depending on your use case. The _setTokenURI is of course optional.
You can also choose to use the Preset available here. A simple contract MyContract is ERC721PresetMinterPauserAutoId will be all you need to get an NFT contract that is preset to have
- minting, pausing and admin roles using the Openzeppelin Access Control mechanism
- enabled pause and unpause functionality
I highly recommend checking out the Openzeppelin 721 contracts and documentation for further details.
How can I receive an NFT in a contract?
As mentioned before, if you use the safeTransferFrom function to send an NFT to a smart contract, it will revert unless the contract specifically added support for receiving 721 tokens
Let's see how you would add this kind of support:
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/IERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721Holder.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/IERC721Receiver.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/introspection/ERC165.sol";
contract Receiver721Example is IERC721Receiver, ERC165, ERC721Holder {
constructor() {
_registerInterface(IERC721Receiver.onERC721Received.selector);
}
function doSomethingWith721Token(IERC721 nftAddress, uint256 tokenId) external {
// do something here
}
}The available ERC721Holder is implementing a basic onERC721Received function which returns the magic value to confirm the support for receiving 721 tokens. Make sure to actually implement a function that can handle the token accordingly.
Use cases for NFT and ERC-721
Most commonly people use the standard for blockchain games like the original Cryptokitties. It's well suited for this case as you can easily have in-game items represented as NFT. The additional metadata file is particularly useful to add more in-game information to the NFT.
But the usage goes beyond just games, and we are seeing it come into existence more and more now. Take a look at Rarible to see many other use cases. Digital art seems to be a major driver for NFT's at this point. Domains can be purchased or utilities, e.g., something that gives you digital access rights. More marketplaces include OpenSea, SuperRare, and Axie Infinity.
I find those in the space of Defi very interesting. For example you can purchase certain insurances, e.g., using yinsure (from yearn). The NFT gives you the permission for the defined insurance and claims are handled and paid out by the decentralized governance.
We even considered using the 721 standard for our derivatives as tokenized positions at Injective Protocol, however we ultimately decided against it as it wasn't a perfect fit. In theory it would have been possible though and might be something to add later on if people show interest.
In general any uniquely identifying possession could be turned into an NFT. We will see much more use cases in the future.
Future and Interchain NFT's
As mentioned we will probably see much more use cases for NFT's in the future on Ethereum and other blockchains. There is a new standard in the making called Interchain NFTs. The Interchain Foundation from the Cosmos universe is developing this standard for multiple networks dealing with 721 NFTs. It will be a useful standard for any blockchain once they have working bridges.
What are you looking forward to? Do you own any NFT's? Have you developed your own?
Let me know in the comments.
Solidity Developer
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Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...
Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...