Refactoring function ownerOf in ERC721.sol

Check the presence of a token using the function _exists. Now the ownerOf function corresponds to the getApproved function.

function ownerOf(uint256 tokenId) public view returns (address) {
        require(_exists(tokenId));
        return _tokenOwner[tokenId];
}

[nventuro]

Ahh, I see. For reference for everyone else, these are the functions being discussed:

function ownerOf(uint256 tokenId) public view returns (address) {
    address owner = _tokenOwner[tokenId];
    require(owner != address(0));
    return owner;
}

function _exists(uint256 tokenId) internal view returns (bool) {
    address owner = _tokenOwner[tokenId];
    return owner != address(0);
}

Yes, I did think of that when revising ownerOf recently, but unfortunately due to how Solidity’s codegen works, that would mean performing an extra SLOAD (since _exists doesn’t return the owner). I think ownerOf is short and readable enough that the benefit of performing that deduplication may not be warranted. @frangio thoughts on this?

[frangio]

I agree with @nventuro that the code is short enough that we should prioritize the cheaper implementation.