ERC-721 (Non-Fungible Token)

The ERC-721 standard introduces a non-fungible token (NFT) in which each issued token is unique and distinct from others. This standard defines functions and events that enable the creation, ownership, and transfer of non-fungible assets.

Note:ERC-721 Token addresses refer to full Hedera Token Service (HTS) fungible token entities. These tokens can be fully managed by HTS API calls. Additionally, by utilizing IERC721 interfaces or system contract functions, these tokens can also be managed by smart contracts on Hedera.

Supported Functions

From Interface ERC-721

ownerOf

function ownerOf(uint256 _tokenId) external view returns (address)

Returns the account ID of the specified HTS token owner. The _tokenId is the Hedera serial number of the NFT.

approve

function approve(address _approved, uint256 _tokenId) external payable

Gives the spender permission to transfer a token (_tokenId) to another account from the owner. The approval is cleared when the token is transferred. The _tokenId is the Hedera serial number of the NFT.

This works by creating a synthetic CryptoApproveAllowanceTransaction with payer - the account that called the precompile (the message sender property of the message frame in the EVM).

If the spender address is 0, this creates a CryptoDeleteAllowanceTransaction instead and removes any allowances previously approved on the token.

Fires an approval event with the following signature when executed:

event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

setApprovalForAll

function setApprovalForAll(address _operator, bool _approved) external

Approve or remove an operator as an operator for the caller. Operators can call transferFrom for any token owned by the caller.

This works by creating a synthetic CryptoApproveAllowanceTransaction with payer - the account that called the precompile (the message sender property of the message frame in the EVM).

getApproved

function getApproved(uint256 _tokenId) external view returns (address)

Returns the account approved for the specified _tokenId. The _tokenId is the Hedera serial number of the NFT.

This works by loading the SPENDER property of the token from the NFTs ledger.

isApprovedForAll

function isApprovedForAll(address _owner, address _operator) external view returns (bool)

Returns if the operator is allowed to manage all of the assets of owner.

This works by loading the APPROVE_FOR_ALL_NFTS_ALLOWANCES property of the owner account and verifying if the list of approved for all accounts contains the account id of the operator.

transferFrom

function transferFrom(address _from, address _to, uint256 _tokenId) external payable

Transfers a token (_tokenId) from a Hedera account (from) to another Hedera account (to) in Solidity format. The _tokenId is the Hedera serial number of the NFT.

This works by creating a synthetic CryptoTransferTransaction with nft token transfers with the is_approval property set to true.

From Interface ERC721Metadata

name

function name() external view returns (string _name)

Returns the name of the HTS non-fungible token.

symbol

function symbol() external view returns (string _symbol)

Returns the symbol of the HTS non-fungible token.

tokenURI

function tokenURI(uint256 _tokenId) external view returns (string)

Returns the token metadata of the HTS non-fungible token. This corresponds to the NFT metadata field when minting an NFT using HTS. The _tokenId is the Hedera serial number of the NFT.

From Interface ERC721Enumerable

totalSupply

function totalSupply() external view returns (uint256)

Returns the total supply of the HTS non-fungible token.

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Unsupported Functions

The following ERC-721 operations will not be natively supported on Hedera and will return a failure if they're called. Advanced functionality is achievable only through custom implementations within smart contracts.

From interface ERC-721

safeTransferFrom

function safeTransferFrom(address token, address from, address to, uint256 tokenId)

From interface ERC721Enumerable

tokenByIndex

function tokenByIndex(uint256 _index) external view returns (uint256)

tokenOfOwnerByIndex

function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256)

All semantics of Interface ERC721TokenReceiver

• Existing Hedera token association rules will take the place of such checks.

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Additional References

• HIP-376
• HIP-218
• EIP-721