Introduction
Today I’m going to make your life a little bit easier by explaining how to create an erc721 non-fungible token(nft) smart contract. We will use the example that I built on my GitHub. Document(DQMT) is a way for anyone to create an immutable record of ideas,items/belongs, poetry and whatever else your heart desires. We accomplish this by asking the user for three pieces of information. 1) Type, 2) Title and 3) Description.
Type - When we were creating the concept of Document we wanted to insure we had a way to keep track of the type of token. In our case we chose type 1 and 2. Type 1 being and idea and type 2 being an item/belongings, but in theory you can use any number to identify the type.
Title - This should be self-explanatory. Give your token a title. Name it whatever you want!
Description - This is were you get to shine. Describe your Idea or Item in this section.
Writing the Contract
Lets get right to it and create the contract. I have intentionally left out some of the security features I implemented within my completed solidity contract. If you would like to see the full contract please see Qwoyn Github. We will be using Remix as our IDE.
Once you have remix open please clear the sample ballot contract and enter:
/**
* @title SafeMath
* @dev Math operations with safety checks that throw on error
*/
library SafeMath {
/**
* @dev Multiplies two numbers, throws on overflow.
*/
function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
// Gas optimization: this is cheaper than asserting 'a' not being zero, but the
// benefit is lost if 'b' is also tested.
// See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
if (a == 0) {
return 0;
}
c = a * b;
assert(c / a == b);
return c;
}
/**
* @dev Integer division of two numbers, truncating the quotient.
*/
function div(uint256 a, uint256 b) internal pure returns (uint256) {
// assert(b > 0); // Solidity automatically throws when dividing by 0
// uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold
return a / b;
}
/**
* @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
*/
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
assert(b <= a);
return a - b;
}
/**
* @dev Adds two numbers, throws on overflow.
*/
function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
c = a + b;
assert(c >= a);
return c;
}
}
Add the ERC165 interface:
pragma solidity ^0.4.24;
/** * @title ERC165 * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md */interface ERC165 { /** * @notice Query if a contract implements an interface * @param _interfaceId The interface identifier, as specified in ERC-165 * @dev Interface identification is specified in ERC-165. This function * uses less than 30,000 gas. */
function supportsInterface(bytes4 _interfaceId)
external
view
returns (bool);}
Here we have defined our compiler version `pragma solidity ^0.4.24;` and ERC165 Interface.
Next add:
/** * @title ERC721 token receiver interface * @dev Interface for any contract that wants to support safeTransfers * from ERC721 asset contracts. */contract ERC721Receiver { /** * @dev Magic value to be returned upon successful reception of an NFT * Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`, * which can be also obtained as `ERC721Receiver(0).onERC721Received.selector` */ bytes4 internal constant ERC721_RECEIVED = 0x150b7a02; /** * @notice Handle the receipt of an NFT * @dev The ERC721 smart contract calls this function on the recipient * after a `safetransfer`. This function MAY throw to revert and reject the * transfer. Return of other than the magic value MUST result in the * transaction being reverted. * Note: the contract address is always the message sender. * @param _operator The address which called `safeTransferFrom` function * @param _from The address which previously owned the token * @param _tokenId The NFT identifier which is being transfered * @param _data Additional data with no specified format * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))` */
function onERC721Received(
address _operator,
address _from,
uint256 _tokenId,
bytes _data )
public returns(bytes4);}
This is the ERC721 token interface for supporting safe transfers. At this time your code should like something like this:
pragma solidity ^0.4.24;
/** * @title ERC165 * @dev https://github.com/ethereum/EIPs/blob/master/EIPS/eip-165.md */interface ERC165 { /** * @notice Query if a contract implements an interface * @param _interfaceId The interface identifier, as specified in ERC-165 * @dev Interface identification is specified in ERC-165. This function * uses less than 30,000 gas. */
function supportsInterface(bytes4 _interfaceId) external view returns (bool);}
/** * @title ERC721 token receiver interface * @dev Interface for any contract that wants to support safeTransfers * from ERC721 asset contracts. */contract ERC721Receiver { /** * @dev Magic value to be returned upon successful reception of an NFT * Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`, * which can be also obtained as `ERC721Receiver(0).onERC721Received.selector` */ bytes4 internal constant ERC721_RECEIVED = 0x150b7a02; /** * @notice Handle the receipt of an NFT * @dev The ERC721 smart contract calls this function on the recipient * after a `safetransfer`. This function MAY throw to revert and reject the * transfer. Return of other than the magic value MUST result in the * transaction being reverted. * Note: the contract address is always the message sender. * @param _operator The address which called `safeTransferFrom` function * @param _from The address which previously owned the token * @param _tokenId The NFT identifier which is being transfered * @param _data Additional data with no specified format * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))` */
function onERC721Received( address _operator,
address _from,
uint256 _tokenId,
bytes _data
)
public returns(bytes4);}
We’ve initialized our interfaces now let’s add the address utils
/**
* Utility library of inline functions on addresses
*/
library AddressUtils {
/**
* Returns whether the target address is a contract
* @dev This function will return false if invoked during the constructor of a contract,
* as the code is not actually created until after the constructor finishes.
* @param addr address to check
* @return whether the target address is a contract
*/
function isContract(address addr) internal view returns (bool) {
uint256 size;
// XXX Currently there is no better way to check if there is a
// contract in an address
// than to check the size of the code at that address.
// See https://ethereum.stackexchange.com/a/14016/36603
// for more details about how this works.
// TODO Check this again before the Serenity release, because
// all addresses will be
// contracts then.
// solium-disable-next-line security/no-inline-assembly
assembly { size := extcodesize(addr) }
return size > 0;
}
Here we will add the ownable contract so we have the ability to transfer ownership of the contract.
contract Ownable {
address public owner;
event OwnershipRenounced(address indexed previousOwner);
event OwnershipTransferred(
address indexed previousOwner,
address indexed newOwner
);
/**
* @dev The Ownable constructor sets the original `owner` of the contract to the sender
* account.
*/
constructor() public {
owner = msg.sender;
}
/**
* @dev Throws if called by any account other than the owner.
*/
modifier onlyOwner() {
require(msg.sender == owner);
_;
}
/**
* @dev Allows the current owner to relinquish control of the contract.
* @notice Renouncing to ownership will leave the contract without an owner.
* It will not be possible to call the functions with the `onlyOwner`
* modifier anymore.
*/
function renounceOwnership() public onlyOwner {
emit OwnershipRenounced(owner);
owner = address(0);
}
/**
* @dev Allows the current owner to transfer control of the contract to a newOwner.
* @param _newOwner The address to transfer ownership to.
*/
function transferOwnership(address _newOwner) public onlyOwner {
_transferOwnership(_newOwner);
}
/**
* @dev Transfers control of the contract to a newOwner.
* @param _newOwner The address to transfer ownership to.
*/
function _transferOwnership(address _newOwner) internal {
require(_newOwner != address(0));
emit OwnershipTransferred(owner, _newOwner);
owner = _newOwner;
}
}
Next we will add the interface with lookup to implement erc165 using a lookup table.
/**
* @title SupportsInterfaceWithLookup
* @author Matt Condon (@shrugs)
* @dev Implements ERC165 using a lookup table.
*/
contract SupportsInterfaceWithLookup is ERC165 {
bytes4 public constant InterfaceId_ERC165 = 0x01ffc9a7;
/**
* 0x01ffc9a7 ===
* bytes4(keccak256('supportsInterface(bytes4)'))
*/
/**
* @dev a mapping of interface id to whether or not it's supported
*/
mapping(bytes4 => bool) internal supportedInterfaces;
/**
* @dev A contract implementing SupportsInterfaceWithLookup
* implement ERC165 itself
*/
constructor()
public
{
_registerInterface(InterfaceId_ERC165);
}
/**
* @dev implement supportsInterface(bytes4) using a lookup table
*/
function supportsInterface(bytes4 _interfaceId)
external
view
returns (bool)
{
return supportedInterfaces[_interfaceId];
}
/**
* @dev private method for registering an interface
*/
function _registerInterface(bytes4 _interfaceId)
internal
{
require(_interfaceId != 0xffffffff);
supportedInterfaces[_interfaceId] = true;
}
}
The contracts above are not required but help us achieve the functionality we are looking for. The next part we are adding is the ERC721 standard, this interface is required in order to be considered part of the standard. Here is what it looks like please add it to your contract. We’ve changed it up slightly but you should be able to get the idea. To get more information about the standard please visit ERC721.ORG.
``
/**
* @title ERC721 Non-Fungible Token Standard basic interface
* @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
*/
contract ERC721Basic is ERC165 {
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) public view returns (uint256 _balance);
function ownerOf(uint256 _tokenId) public view returns (address _owner);
function exists(uint256 _tokenId) public view returns (bool _exists);
function approve(address _to, uint256 _tokenId) public;
function getApproved(uint256 _tokenId)
public view returns (address _operator);
function setApprovalForAll(address _operator, bool _approved) public;
function isApprovedForAll(address _owner, address _operator)
public view returns (bool);
function transferFrom(address _from, address _to, uint256 _tokenId) public;
function safeTransferFrom(address _from, address _to, uint256 _tokenId)
public;
function safeTransferFrom(
address _from,
address _to,
uint256 _tokenId,
bytes _data
)
public;
}
/**
* @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
* @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
*/
contract ERC721Enumerable is ERC721Basic {
function totalSupply() public view returns (uint256);
function tokenOfOwnerByIndex(
address _owner,
uint256 _index
)
public
view
returns (uint256 _tokenId);
function tokenByIndex(uint256 _index) public view returns (uint256);
}
/**
* @title ERC-721 Non-Fungible Token Standard, optional metadata extension
* @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
*/
contract ERC721Metadata is ERC721Basic {
function name() external view returns (string _name);
function symbol() external view returns (string _symbol);
function tokenURI(uint256 _tokenId) public view returns (string);
}
/**
* @title ERC-721 Non-Fungible Token Standard, full implementation interface
* @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
*/
contract ERC721 is ERC721Basic, ERC721Enumerable, ERC721Metadata {
}
/**
* @title ERC721 Non-Fungible Token Standard basic implementation
* @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
*/
contract ERC721BasicToken is SupportsInterfaceWithLookup, ERC721Basic {
bytes4 private constant InterfaceId_ERC721 = 0x80ac58cd;
/*
* 0x80ac58cd ===
* bytes4(keccak256('balanceOf(address)')) ^
* bytes4(keccak256('ownerOf(uint256)')) ^
* bytes4(keccak256('approve(address,uint256)')) ^
* bytes4(keccak256('getApproved(uint256)')) ^
* bytes4(keccak256('setApprovalForAll(address,bool)')) ^
* bytes4(keccak256('isApprovedForAll(address,address)')) ^
* bytes4(keccak256('transferFrom(address,address,uint256)')) ^
* bytes4(keccak256('safeTransferFrom(address,address,uint256)')) ^
* bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))
*/
bytes4 private constant InterfaceId_ERC721Exists = 0x4f558e79;
/*
* 0x4f558e79 ===
* bytes4(keccak256('exists(uint256)'))
*/
using SafeMath for uint256;
using AddressUtils for address;
// Equals tobytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))// which can be also obtained asERC721Receiver(0).onERC721Received.selectorbytes4 private constant ERC721_RECEIVED = 0x150b7a02;
// Mapping from token ID to owner
mapping (uint256 => address) internal tokenOwner;
// Mapping from token ID to approved address
mapping (uint256 => address) internal tokenApprovals;
// Mapping from owner to number of owned token
mapping (address => uint256) internal ownedTokensCount;
// Mapping from owner to operator approvals
mapping (address => mapping (address => bool)) internal operatorApprovals;
/**
* @dev Guarantees msg.sender is owner of the given token
* @param _tokenId uint256 ID of the token to validate its ownership belongs to msg.sender
*/
modifier onlyOwnerOf(uint256 _tokenId) {
require(ownerOf(_tokenId) == msg.sender);
_;
}
/**
* @dev Checks msg.sender can transfer a token, by being owner, approved, or operator
* @param _tokenId uint256 ID of the token to validate
*/
modifier canTransfer(uint256 _tokenId) {
require(isApprovedOrOwner(msg.sender, _tokenId));
_;
}
constructor()
public
{
// register the supported interfaces to conform to ERC721 via ERC165
_registerInterface(InterfaceId_ERC721);
_registerInterface(InterfaceId_ERC721Exists);
}
/**
* @dev Gets the balance of the specified address
* @param _owner address to query the balance of
* @return uint256 representing the amount owned by the passed address
*/
function balanceOf(address _owner) public view returns (uint256) {
require(_owner != address(0));
return ownedTokensCount[_owner];
}
/**
* @dev Gets the owner of the specified token ID
* @param _tokenId uint256 ID of the token to query the owner of
* @return owner address currently marked as the owner of the given token ID
*/
function ownerOf(uint256 _tokenId) public view returns (address) {
address owner = tokenOwner[_tokenId];
require(owner != address(0));
return owner;
}
/**
* @dev Returns whether the specified token exists
* @param _tokenId uint256 ID of the token to query the existence of
* @return whether the token exists
*/
function exists(uint256 _tokenId) public view returns (bool) {
address owner = tokenOwner[_tokenId];
return owner != address(0);
}
/**
* @dev Approves another address to transfer the given token ID
* The zero address indicates there is no approved address.
* There can only be one approved address per token at a given time.
* Can only be called by the token owner or an approved operator.
* @param _to address to be approved for the given token ID
* @param _tokenId uint256 ID of the token to be approved
*/
function approve(address _to, uint256 _tokenId) public {
address owner = ownerOf(_tokenId);
require(_to != owner);
require(msg.sender == owner || isApprovedForAll(owner, msg.sender));
tokenApprovals[_tokenId] = _to;
emit Approval(owner, _to, _tokenId);
}
/**
* @dev Gets the approved address for a token ID, or zero if no address set
* @param _tokenId uint256 ID of the token to query the approval of
* @return address currently approved for the given token ID
*/
function getApproved(uint256 _tokenId) public view returns (address) {
return tokenApprovals[_tokenId];
}
/**
* @dev Sets or unsets the approval of a given operator
* An operator is allowed to transfer all tokens of the sender on their behalf
* @param _to operator address to set the approval
* @param _approved representing the status of the approval to be set
*/
function setApprovalForAll(address _to, bool _approved) public {
require(_to != msg.sender);
operatorApprovals[msg.sender][_to] = _approved;
emit ApprovalForAll(msg.sender, _to, _approved);
}
/**
* @dev Tells whether an operator is approved by a given owner
* @param _owner owner address which you want to query the approval of
* @param _operator operator address which you want to query the approval of
* @return bool whether the given operator is approved by the given owner
*/
function isApprovedForAll(
address _owner,
address _operator
)
public
view
returns (bool)
{
return operatorApprovals[_owner][_operator];
}
/**
* @dev Transfers the ownership of a given token ID to another address
* Usage of this method is discouraged, usesafeTransferFromwhenever possible
* Requires the msg sender to be the owner, approved, or operator
* @param _from current owner of the token
* @param _to address to receive the ownership of the given token ID
* @param _tokenId uint256 ID of the token to be transferred
*/
function transferFrom(
address _from,
address _to,
uint256 _tokenId
)
public
canTransfer(_tokenId)
{
require(_from != address(0));
require(_to != address(0));
clearApproval(_from, _tokenId);
removeTokenFrom(_from, _tokenId);
addTokenTo(_to, _tokenId);
emit Transfer(_from, _to, _tokenId);
}
/**
* @dev Safely transfers the ownership of a given token ID to another address
* If the target address is a contract, it must implementonERC721Received,
* which is called upon a safe transfer, and return the magic value
*bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
* the transfer is reverted.
*
* Requires the msg sender to be the owner, approved, or operator
* @param _from current owner of the token
* @param _to address