checkOnERC721Received

checkOnERC721Received is a mechanism within the ERC-721 standard used to verify that a token recipient can properly handle ERC-721 tokens, especially when the recipient is a smart contract. In typical implementations, it is an internal function invoked during safe transfer operations to ensure compatibility and prevent tokens from being sent to contracts that cannot manage them.

During a safe transfer, such as safeTransferFrom, the contract first transfers the token and then calls checkOnERC721Received with parameters including the operator, the previous owner, the recipient, the tokenId, and any data. If the recipient address is not a contract, the check passes automatically. If the recipient is a contract, it must implement the IERC721Receiver interface and specifically return the magic value produced by onERC721Received. If the call fails or the returned value is incorrect, the transfer reverts.

The primary purpose of this check is to prevent tokens from being permanently locked in contracts that do not recognize or handle ERC-721 tokens. By requiring a contract to implement onERC721Received and return the expected value, the ERC-721 contract can safely transfer tokens to contracts that can interact with them.

Implementation notes: various ERC-721 implementations expose a function named _checkOnERC721Received (or a similarly named internal helper) that performs a try/catch call to IERC721Receiver(to).onERC721Received and validates the returned selector. If the recipient is a non-contract, the function returns true without external calls. If the recipient contract does not implement the interface correctly, the transfer reverts with an error.

See also: ERC-721 standard, IERC721Receiver, SafeTransferFrom, and common libraries like OpenZeppelin's ERC721.