> For the complete documentation index, see [llms.txt](https://developers.aixchain.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developers.aixchain.com/aixc-and-arc-token/arc-721/protocol-interface.md). # Protocol Interface ## ARC-721 Standard A standard interface allows applications to track and transfer NFTs on Tron. Simple ARC-721 smart contracts list as below to track a large number of NFTs. ARC-20 token standard is insufficient for handling NFTs due to each token in ARC-721 being unique. The standard of ARC-721 is inspiring on Tron. It plays an important role as well as ARC-20. ## MUST Implemented Interfaces Every ARC-721 compliant contract must implement the ARC721 and ARC165 interfaces solidity ```javascript pragma solidity ^0.4.20; interface ARC721 { 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) 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); } interface ARC165 { function supportsInterface(bytes4 interfaceID) external view returns (bool); } ``` **balanceOf(address \_owner)**\ Returns the number of NFTs owned by the specified account **ownerOf(uint256 \_tokenId)**\ Returns the owner of the specified NFT **safeTransferFrom(address \_from, address \_to, uint256 \_tokenId, bytes data)**\ Transfer ownership of an NFT **safeTransferFrom(address \_from, address \_to, uint256 \_tokenId)**\ Transfer ownership of an NFT **transferFrom(address \_from, address \_to, uint256 \_tokenId)**\ Transfer the ownership of an NFT (the caller must confirm whether the \_to address can receive the NFT normally, otherwise the NFT will be lost) **approve(address \_approved, uint256 \_tokenId)**\ Grant other people control of an NFT **setApprovalForAll(address \_operator, bool \_approved)**\ Grant/recover control of all NFTs by a third party (\_operator) **getApproved(uint256 \_tokenId)**\ Query the authorization of a certain NFT **isApprovedForAll(address \_owner, address \_operator)**\ Query whether the operator is the authorized address of the owner **supportsInterface(bytes4 interfaceID)**\ Query whether a certain interface is supported (interfaceID) **event Approval(address indexed \_owner, address indexed \_approved, uint256 indexed \_tokenId)**\ Approval event will be triggered after Approval is successful **event Transfer(address indexed \_from, address indexed \_to, uint256 indexed \_tokenId)**\ Successful transferFrom and safeTransferFrom will trigger the Transfer event **event ApprovalForAll(address indexed \_owner, address indexed \_operator, bool \_approved)**\ ApprovalForAll event will be triggered after setApprovalForAll succeeds A wallet/broker/auction application MUST implement the wallet interface if it will accept safe transfers. solidity ```javascript interface ARC721TokenReceiver { function onARC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4); } ``` **onARC721Received(address \_operator, address \_from, uint256 \_tokenId, bytes \_data)** Works with the `safeTransferFrom` method, when \_to is the contract address, you need to call this method and check the return value. If the return value is not bytes4(keccak256("onARC721Received(address,address,uint256,bytes)")) an exception will be thrown. A smart contract that can receive NFT must implement the ARC721TokenReceiver interface. > #### 📘 > > The hash of bytes4(keccak256("onARC721Received(address,address,uint256,bytes))) is different from the Ethereum version bytes4(keccak256("onERC721Received(address,address,uint256,bytes))). With the return value of function `onARC721Received`, please use 0x5175f878 instead of 0x150b7a02. ## OPTIONAL Metadata Extension Interface The metadata extension is OPTIONAL for ARC-721 smart contracts. This allows your smart contract to be interrogated for its name and for details about the assets which your NFTs represent. solidity ```javascript interface ARC721Metadata { function name() external view returns (string _name); function symbol() external view returns (string _symbol); function tokenURI(uint256 _tokenId) external view returns (string); } ``` **name()**\ Returns the contract name **symbol()**\ Returns the contract symbol **tokenURI(uint256 \_tokenId)**\ Returns the URI of the external file corresponding to \_tokenId. External resource files need to include names, descriptions and pictures. ## OPTIONAL Enumeration Extension Interface The enumeration extension is OPTIONAL for ARC-721 smart contracts. This allows your contract to publish its full list of NFTs and make them discoverable. solidity ```javascript interface ARC721Enumerable { function totalSupply() external view returns (uint256); function tokenByIndex(uint256 _index) external view returns (uint256); function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256); } ``` **totalSupply()**\ Returns the total amount of NFT **tokenByIndex(uint256 \_index)**\ Returns the corresponding tokenId through \_index **tokenOfOwnerByIndex(address \_owner, uint256 \_index)**\ Returns the tokenId corresponding to the index in the NFT list owned by the owner --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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