ERC721

ERC721 is a standard interface for non-fungible tokens (NFTs) on the Ethereum blockchain. It defines how unique tokens can be owned, transferred, and managed, enabling the creation and interaction of digital assets that are distinct from one another rather than interchangeable.

Core functionality is provided through a set of required functions and events. The essential methods include balanceOf(owner) to query how many tokens an address owns, ownerOf(tokenId) to identify the owner of a specific token, transferFrom(from, to, tokenId) and safeTransferFrom(from, to, tokenId) for transfers, and approve(to, tokenId) or setApprovalForAll(operator, approved) to grant permissions. getApproved(tokenId) returns a specific approved address, while isApprovedForAll(owner, operator) checks bulk approvals. The core events are Transfer and Approval, with an ApprovalForAll event for bulk permissions. Tokens are represented by a uint256 tokenId and are non-fungible by design.

Metadata and extensibility are supported via optional extensions. ERC721Metadata adds name(), symbol(), and tokenURI(tokenId) to provide human-readable information and token-specific metadata. Some projects also implement ERC721Enumerable to enable enumeration of tokens owned by an address and across the total supply.

Compliance and safety are enabled by ERC165, signaling support for the ERC721 interface. Transfers to smart contracts use safeTransferFrom to ensure recipients can handle NFTs, reducing the risk of tokens becoming unrecoverable.

ERC721 has become the de facto standard for many NFT use cases, including digital art, collectibles, and tokenized assets, and is widely supported by wallets, marketplaces, and developers.