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I was fiddling with OpenZeppelin ERC721 contract and created my own contract inheriting from the ERC721.sol. I tried to change the _safeMint method, so I copied it from the parent contract and this require line was raising an error:

function _safeMint(address to, uint256 tokenId, bytes memory _data) internal virtual {
    // Some extra logic here while keeping the next line exactly as in the parent contract
    require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
}

Error:

contracts/nft.sol:98:17: DeclarationError: Undeclared identifier. 
require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer"); ^--------------------^

I believe this was due the _checkOnERC721Received was defined as private in the parent contract, and this made it unavailable to the child contract. So I decide to copy the _checkOnERC721Received method into my contract to solve the issue. However it raised another error:

contracts/nft.sol:114:5: TypeError: Overriding function is missing "override" specifier. 
function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data) ^ (Relevant source part starts here and spans across multiple lines). 
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.4.0/contracts/token/ERC721/ERC721.sol:435:5: Overridden function is here: 
function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data) ^ (Relevant source part starts here and spans across multiple lines).

And if I add the override param I now get:

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.4.0/contracts/token/ERC721/ERC721.sol:435:5: TypeError: Trying to override non-virtual function.
Did you forget to add "virtual"? function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data) ^ (Relevant source part starts here and spans across multiple lines). 

contracts/nft.sol:114:5: Overriding function is here: 
function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data) ^ (Relevant source part starts here and spans across multiple lines).

So it seems I can't override that method? How could I change the _safeMint logic in an child contract, while keeping this line https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.4.0/contracts/token/ERC721/ERC721.sol#L318 ? Is the only way by creating the whole contract without inheritance?

The questions is for learning purposes, I'm not trying to do anything specific.

1 Answer 1

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Two things you can do change _safeMint behavior

  • Override _safeMint and call its parrent

    contract MyToken is ERC721 {
        function _safeMint(address to, uint256 tokenId, bytes memory _data) internal override {
            /* Can do something here and modify input parameters */
            super._safeMint(to, tokenId, _data);
            /* Can do something else here */
        }
    }
    
  • Override _mint, in the current implementation _safeMint only calls _mint

    contract MyToken is ERC721 {
        function _mint(address to, uint256 tokenId) internal override {
            /* Can do something here and modify input parameters */
            super.mint(to, tokenId);
            /* Can do something else here */
        }
    }
    

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