Check require() before or after

Smart Contracts
1

Here is the _safeMint from ERc721.sol the implementation of ERC-721

function _safeMint(address to, uint256 tokenId, bytes memory _data) internal virtual {
     _mint(to, tokenId);
     require(
          _checkOnERC721Received(address(0), to, tokenId, _data),
          "ERC721: transfer to non ERC721Receiver implementer"
     );
}

I understand it is calling the _mint and then checking the condition of onERC721Received. My question is why don't you check the condition first and then call the _mint()? Any reason for checking the condition after calling?

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2

It don't really matter at the end of the day. Any failure will revert the whole transaction. And it makes logic sense that a token is minted to the recipient first and check second if the receiving end meets the requirement.

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3

on this particular case, the call to _checkOnERC721Received needs to come after the mint/safeTransfer call (check the function code for reference) because it checks if the receiver is a contract and calls the function onERC721Received on that contract. Logically you can only 'tell' the contract it has received a token after it is minted/transfered

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