function transferFrom(address from, address to, uint256 id) public { require(from == _ownerOf[id], "from != owner"); require(to != address(0), "transfer to zero address"); require(_isApprovedOrOwner(from, msg.sender, id), "not authorized"); _balanceOf[from]--; _balanceOf[to]++; _ownerOf[id] = to; delete _approvals[id]; emit Transfer(from, to, id); } 0.7.1