ERC721s

ERC-721 is a token standard on the Ethereum blockchain for representing non-fungible tokens (NFTs). Each token is identified by a unique tokenId and has a specific owner. Unlike fungible tokens such as ERC-20, ERC-721 tokens are not interchangeable and can carry distinct metadata and value.

A compliant ERC-721 contract implements a core set of functions and events defined by the IERC721 interface, including balanceOf(owner) to report how many tokens an address holds, ownerOf(tokenId) to identify the owner, safeTransferFrom and transferFrom for transfers, approve to grant approval for a single token, getApproved to query the approved address, setApprovalForAll and isApprovedForAll to manage operator approvals. It also emits Transfer, Approval, and ApprovalForAll events. The contract typically also implements IERC165 to declare support for ERC-721.

Optional extensions add more features. IERC721Metadata introduces name, symbol, and tokenURI(tokenId) for token-specific metadata. IERC721Enumerable provides enumeration helpers such as totalSupply, tokenOfOwnerByIndex, and tokenByIndex, though this extension is not mandatory and not all implementations include it.

Storage of metadata is commonly done via tokenURI, which returns a URI pointing to a JSON metadata document describing the asset. Many projects rely on well-audited implementations (for example from OpenZeppelin) to ensure security and interoperability with wallets and marketplaces.

Common use cases include digital art, collectibles, in-game items, and tokenized real-world assets. ERC-721 is often contrasted with ERC-1155, which can support both fungible and non-fungible tokens and can be more gas-efficient for certain batch operations. Security considerations include using safeTransferFrom for contract receivers and carefully managing approvals.