# ARC-721 ## 1 ARC-721 Protocol Standard ARC-721 is a set of standard interfaces, for issuing non-fungible tokens(NFT) on the AIXChain network. ARC-721 is fully compatible with ERC-721. ### 1.1 ARC-721 Smart Contract Interface Implementation Every ARC-721 compliant contract must implement the ARC721 and ARC165 interfaces. Other extension interfaces can be implemented according to specific business requirements. #### 1.1.1 ARC-721 & ARC-165 Interfaces ```javascript pragma solidity ^0.4.20; interface ARC721 { // Returns the number of NFTs owned by the given account function balanceOf(address _owner) external view returns (uint256); //Returns the owner of the given NFT function ownerOf(uint256 _tokenId) external view returns (address); //Transfer ownership of NFT function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable; //Transfer ownership of NFT function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable; //Transfer ownership of NFT function transferFrom(address _from, address _to, uint256 _tokenId) external payable; //Grants address ‘_approved’ the authorization of the NFT ‘_tokenId’ function approve(address _approved, uint256 _tokenId) external payable; //Grant/recover all NFTs’ authorization of the ‘_operator’ function setApprovalForAll(address _operator, bool _approved) external; //Query the authorized address of NFT function getApproved(uint256 _tokenId) external view returns (address); //Query whether the ‘_operator’ is the authorized address of the ‘_owner’ function isApprovedForAll(address _owner, address _operator) external view returns (bool); //The successful ‘transferFrom’ and ‘safeTransferFrom’ will trigger the ‘Transfer’ Event event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId); //The successful ‘Approval’ will trigger the ‘Approval’ event event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId); //The successful ‘setApprovalForAll’ will trigger the ‘ApprovalForAll’ event event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved); } interface ARC165 { //Query whether the interface ‘interfaceID’ is supported function supportsInterface(bytes4 interfaceID) external view returns (bool); } ``` A wallet/broker/auction application MUST implement the `wallet` interface if it will accept safe transfers. Solidity ```javascript interface ARC721TokenReceiver { //This method will be triggered when the ‘_to’ is the contract address during the ‘safeTransferFrom’ execution, and the return value must be checked, If the return value is not bytes4(keccak256("onTRC721Received(address,address,uint256,bytes)")) throws an exception. The smart contract which can receive NFT must implement the TRC721TokenReceiver interface. function onARC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4); } ``` > The hash of `bytes4(keccak256("onWRC721Received(address,address,uint256,bytes)))` is different from the Ethereum version `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)))`. Please use `0x5175f878` instead of `0x150b7a02`. #### 1.1.2 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 { //Return the token name function name() external view returns (string _name); //Return the token symbol function symbol() external view returns (string _symbol); //Returns the URI of the external file corresponding to ‘_tokenId’. External resource files need to include names, descriptions and pictures. function tokenURI(uint256 _tokenId) external view returns (string); } ``` URI is a URI link describing the \_tokenId asset, pointing to a JSON file that conforms to the ARC721 metadata description structure. When tokens are minted, each token needs to be assigned a unique URI: JSON ```json { "title": "Asset Metadata", "type": "object", "properties": { "name": { "type": "string", "description": "Identifies the asset to which this NFT represents" }, "description": { "type": "string", "description": "Describes the asset to which this NFT represents" }, "image": { "type": "string", "description": "A URI pointing to a resource with mime type image/* representing the asset to which this NFT represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive." } } } ``` #### 1.1.3 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 { //Return the total supply of NFT function totalSupply() external view returns (uint256); //Return the corresponding ‘tokenId’ through ‘_index’ function tokenByIndex(uint256 _index) external view returns (uint256); //Return the ‘tokenId’ corresponding to the index in the NFT list owned by the ‘_owner' function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256); } ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://developers.aixchain.com/aixc-and-arc-token/arc-721.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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