ERC721-Open Zeppelin Smart contract

Question

I hope you are all well :)

I have a question about the _checkOnERC721Received function of the ERC721 smart contract of Openzeppelin.

I don't understand this part:

try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
               return retval == IERC721Receiver.onERC721Received.selector;

What must be implemented in the onERC721Received function?

Thank you for your help

Take care

Pierre

Answer 1

This is specified in https://eips.ethereum.org/EIPS/eip-721#specification

A wallet/broker/auction application MUST implement the wallet interface if it will accept safe transfers.

It can simply implement this function and not do anything. The signature is

function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);

This allows the NFT token contract to ensure that it is sending to a contract that is expecting to receive tokens (i.e. avoiding "stuck" tokens").