Use storage slot for upgradeable contracts

Author: ScreamingHawk

src/contracts/mocks/ERC1155MetadataUpgradeableMock.sol

@@ -72,6 +72,6 @@ contract ERC1155MetadataUpgradeableMockV2 is ERC1155MetadataUpgradeableMock {
   }
 
   function uri(uint256 _id) public override view returns (string memory) {
-    return string(abi.encodePacked(baseURI, _uint2str(idMapping[_id]))); // Removes .json extension, swaps ids
+    return string(abi.encodePacked(baseURI(), _uint2str(idMapping[_id]))); // Removes .json extension, swaps ids
   }
 }

src/contracts/tokens/ERC1155/ERC1155Meta.sol

@@ -294,12 +294,13 @@ contract ERC1155Meta is ERC1155, SignatureValidator {
     // Complete data to pass to signer verifier
     bytes memory fullData = abi.encodePacked(_encMembers, nonce, signedData);
 
-    //Update signature nonce
+    // Verify if _from is the signer
+    require(isValidSignature(_signer, hash, fullData, sig), "ERC1155Meta#_signatureValidation: INVALID_SIGNATURE");
+
+    // Update signature nonce
     nonces[_signer] = nonce + 1;
     emit NonceChange(_signer, nonce + 1);
 
-    // Verify if _from is the signer
-    require(isValidSignature(_signer, hash, fullData, sig), "ERC1155Meta#_signatureValidation: INVALID_SIGNATURE");
     return signedData;
   }
 

src/contracts/tokens/ERC1155PackedBalanceUpgradeable/ERC1155MetaPackedBalanceUpgradeable.sol

@@ -6,7 +6,7 @@ import "../../interfaces/IERC20.sol";
 import "../../interfaces/IERC1155.sol";
 import "../../utils/LibBytes.sol";
 import "../../utils/SignatureValidator.sol";
-
+import '../../utils/StorageSlot.sol';
 
 /**
  * @dev ERC-1155 with native metatransaction methods. These additional functions allow users
@@ -45,7 +45,7 @@ contract ERC1155MetaPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradeable,
   }
 
   // Signature nonce per address
-  mapping (address => uint256) internal nonces;
+  bytes32 constant private _NONCES_SLOT_KEY = keccak256("0xsequence.ERC1155MetaPackedBalanceUpgradeable.nonces");
 
 
   /***********************************|
@@ -227,7 +227,7 @@ contract ERC1155MetaPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradeable,
     );
 
     // Update operator status
-    operators[_owner][_operator] = _approved;
+    _setOperator(_owner, _operator, _approved);
 
     // Emit event
     emit ApprovalForAll(_owner, _operator, _approved);
@@ -284,8 +284,8 @@ contract ERC1155MetaPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradeable,
     (sig, signedData) = abi.decode(_sigData, (bytes, bytes));
 
     // Get current nonce and nonce used for signature
-    uint256 currentNonce = nonces[_signer];        // Lowest valid nonce for signer
-    uint256 nonce = uint256(sig.readBytes32(65));  // Nonce passed in the signature object
+    uint256 currentNonce = _getNonce(_signer);   // Lowest valid nonce for signer
+    uint256 nonce = uint256(sig.readBytes32(65)); // Nonce passed in the signature object
 
     // Verify if nonce is valid
     require(
@@ -300,8 +300,9 @@ contract ERC1155MetaPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradeable,
     bytes memory fullData = abi.encodePacked(_encMembers, nonce, signedData);
 
     //Update signature nonce
-    nonces[_signer] = nonce + 1;
-    emit NonceChange(_signer, nonce + 1);
+    nonce++;
+    _setNonce(_signer, nonce);
+    emit NonceChange(_signer, nonce);
 
     // Verify if _from is the signer
     require(isValidSignature(_signer, hash, fullData, sig), "ERC1155MetaPackedBalance#_signatureValidation: INVALID_SIGNATURE");
@@ -315,7 +316,26 @@ contract ERC1155MetaPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradeable,
   function getNonce(address _signer)
     public view returns (uint256 nonce)
   {
-    return nonces[_signer];
+    return _getNonce(_signer);
+  }
+
+  /**
+   * @notice Returns the current nonce associated with a given address
+   * @param _signer Address to query signature nonce for
+   */
+  function _getNonce(address _signer)
+    internal view returns (uint256 nonce)
+  {
+    return StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_NONCES_SLOT_KEY, _signer))).value;
+  }
+
+  /**
+   * @notice Sets the nonce associated with a given address
+   * @param _signer Address to set signature nonce for
+   * @param _nonce Nonce value to set
+   */
+  function _setNonce(address _signer, uint256 _nonce) internal {
+    StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_NONCES_SLOT_KEY, _signer))).value = _nonce;
   }
 
 

src/contracts/tokens/ERC1155PackedBalanceUpgradeable/ERC1155MintBurnPackedBalanceUpgradeable.sol

@@ -51,7 +51,7 @@ contract ERC1155MintBurnPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradea
       (uint256 bin, uint256 index) = getIDBinIndex(_ids[0]);
 
       // Balance for current bin in memory (initialized with first transfer)
-      uint256 balTo = _viewUpdateBinValue(balances[_to][bin], index, _amounts[0], Operations.Add);
+      uint256 balTo = _viewUpdateBinValue(_getBalance(_to, bin), index, _amounts[0], Operations.Add);
 
       // Number of transfer to execute
       uint256 nTransfer = _ids.length;
@@ -65,8 +65,8 @@ contract ERC1155MintBurnPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradea
         // If new bin
         if (bin != lastBin) {
           // Update storage balance of previous bin
-          balances[_to][lastBin] = balTo;
-          balTo = balances[_to][bin];
+          _setBalance(_to, lastBin, balTo);
+          balTo = _getBalance(_to, bin);
 
           // Bin will be the most recent bin
           lastBin = bin;
@@ -77,7 +77,7 @@ contract ERC1155MintBurnPackedBalanceUpgradeable is ERC1155PackedBalanceUpgradea
       }
 
       // Update storage of the last bin visited
-      balances[_to][bin] = balTo;
+      _setBalance(_to, bin, balTo);
     }
 
     // //Emit event

src/contracts/tokens/ERC1155PackedBalanceUpgradeable/ERC1155PackedBalanceUpgradeable.sol

@@ -6,6 +6,7 @@ import "../../interfaces/IERC1155.sol";
 import "../../utils/Address.sol";
 import "../../utils/ContextUpgradeable.sol";
 import "../../utils/ERC165.sol";
+import '../../utils/StorageSlot.sol';
 
 /**
  * @dev Implementation of Multi-Token Standard contract. This implementation of the ERC-1155 standard
@@ -35,10 +36,10 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
   enum Operations { Add, Sub }
 
   // Token IDs balances ; balances[address][id] => balance (using array instead of mapping for efficiency)
-  mapping (address => mapping(uint256 => uint256)) internal balances;
+  bytes32 constant private _BALANCES_SLOT_KEY = keccak256("0xsequence.ERC1155PackedBalanceUpgradeable.balances");
 
   // Operators
-  mapping (address => mapping(address => bool)) internal operators;
+  bytes32 constant private _OPERATORS_SLOT_KEY = keccak256("0xsequence.ERC1155PackedBalanceUpgradeable.operators");
 
 
   /***********************************|
@@ -140,8 +141,8 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
       (uint256 bin, uint256 index) = getIDBinIndex(_ids[0]);
 
       // Balance for current bin in memory (initialized with first transfer)
-      uint256 balFrom = _viewUpdateBinValue(balances[_from][bin], index, _amounts[0], Operations.Sub);
-      uint256 balTo = _viewUpdateBinValue(balances[_to][bin], index, _amounts[0], Operations.Add);
+      uint256 balFrom = _viewUpdateBinValue(_getBalance(_from, bin), index, _amounts[0], Operations.Sub);
+      uint256 balTo = _viewUpdateBinValue(_getBalance(_to, bin), index, _amounts[0], Operations.Add);
 
       // Last bin updated
       uint256 lastBin = bin;
@@ -152,11 +153,11 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
         // If new bin
         if (bin != lastBin) {
           // Update storage balance of previous bin
-          balances[_from][lastBin] = balFrom;
-          balances[_to][lastBin] = balTo;
+          _setBalance(_from, lastBin, balFrom);
+          _setBalance(_to, lastBin, balTo);
 
-          balFrom = balances[_from][bin];
-          balTo = balances[_to][bin];
+          balFrom = _getBalance(_from, bin);
+          balTo = _getBalance(_to, bin);
 
           // Bin will be the most recent bin
           lastBin = bin;
@@ -168,8 +169,8 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
       }
 
       // Update storage of the last bin visited
-      balances[_from][bin] = balFrom;
-      balances[_to][bin] = balTo;
+      _setBalance(_from, bin, balFrom);
+      _setBalance(_to, bin, balTo);
 
     // If transfer to self, just make sure all amounts are valid
     } else {
@@ -209,7 +210,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
     external override
   {
     // Update operator status
-    operators[_msgSender()][_operator] = _approved;
+    _setOperator(_msgSender(), _operator, _approved);
     emit ApprovalForAll(_msgSender(), _operator, _approved);
   }
 
@@ -222,7 +223,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
   function isApprovedForAll(address _owner, address _operator)
     public override view returns (bool isOperator)
   {
-    return operators[_owner][_operator];
+    return _getOperator(_owner, _operator);
   }
 
 
@@ -244,7 +245,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
 
     //Get bin and index of _id
     (bin, index) = getIDBinIndex(_id);
-    return getValueInBin(balances[_owner][bin], index);
+    return getValueInBin(_getBalance(_owner, bin), index);
   }
 
   /**
@@ -261,7 +262,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
 
     // First values
     (uint256 bin, uint256 index) = getIDBinIndex(_ids[0]);
-    uint256 balance_bin = balances[_owners[0]][bin];
+    uint256 balance_bin = _getBalance(_owners[0], bin);
     uint256 last_bin = bin;
 
     // Initialization
@@ -274,7 +275,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
 
       // SLOAD if bin changed for the same owner or if owner changed
       if (bin != last_bin || _owners[i-1] != _owners[i]) {
-        balance_bin = balances[_owners[i]][bin];
+        balance_bin = _getBalance(_owners[i], bin);
         last_bin = bin;
       }
 
@@ -308,7 +309,7 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
     (bin, index) = getIDBinIndex(_id);
 
     // Update balance
-    balances[_address][bin] = _viewUpdateBinValue(balances[_address][bin], index, _amount, _operation);
+    _setBalance(_address, bin, _viewUpdateBinValue(_getBalance(_address, bin), index, _amount, _operation));
   }
 
   /**
@@ -379,6 +380,26 @@ contract ERC1155PackedBalanceUpgradeable is ContextUpgradeable, IERC1155, ERC165
     return (_binValues >> rightShift) & mask;
   }
 
+  /***********************************|
+  |         Storage Functions         |
+  |__________________________________*/
+
+  function _getBalance(address _owner, uint256 _id) internal view returns (uint256) {
+    return StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_BALANCES_SLOT_KEY, _owner, _id))).value;
+  }
+
+  function _setBalance(address _owner, uint256 _id, uint256 _balance) internal {
+    StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_BALANCES_SLOT_KEY, _owner, _id))).value = _balance;
+  }
+
+  function _getOperator(address _owner, address _operator) internal view returns (bool) {
+    return StorageSlot.getBooleanSlot(keccak256(abi.encodePacked(_OPERATORS_SLOT_KEY, _owner, _operator))).value;
+  }
+
+  function _setOperator(address _owner, address _operator, bool _approved) internal {
+    StorageSlot.getBooleanSlot(keccak256(abi.encodePacked(_OPERATORS_SLOT_KEY, _owner, _operator))).value = _approved;
+  }
+
 
   /***********************************|
   |          ERC165 Functions         |

src/contracts/tokens/ERC1155Upgradeable/ERC1155MetaUpgradeable.sol

@@ -6,6 +6,7 @@ import "../../interfaces/IERC20.sol";
 import "../../interfaces/IERC1155.sol";
 import "../../utils/LibBytes.sol";
 import "../../utils/SignatureValidator.sol";
+import '../../utils/StorageSlot.sol';
 
 /**
  * @dev ERC-1155 with native metatransaction methods. These additional functions allow users
@@ -41,8 +42,7 @@ contract ERC1155MetaUpgradeable is ERC1155Upgradeable, SignatureValidator {
   }
 
   // Signature nonce per address
-  mapping (address => uint256) internal nonces;
-
+  bytes32 constant private _NONCES_SLOT_KEY = keccak256("0xsequence.ERC1155MetaUpgradeable.nonces");
 
   /***********************************|
   |               Events              |
@@ -223,7 +223,7 @@ contract ERC1155MetaUpgradeable is ERC1155Upgradeable, SignatureValidator {
     );
 
     // Update operator status
-    operators[_owner][_operator] = _approved;
+    _setOperator(_owner, _operator, _approved);
 
     // Emit event
     emit ApprovalForAll(_owner, _operator, _approved);
@@ -279,8 +279,8 @@ contract ERC1155MetaUpgradeable is ERC1155Upgradeable, SignatureValidator {
     (sig, signedData) = abi.decode(_sigData, (bytes, bytes));
 
     // Get current nonce and nonce used for signature
-    uint256 currentNonce = nonces[_signer];        // Lowest valid nonce for signer
-    uint256 nonce = uint256(sig.readBytes32(65));  // Nonce passed in the signature object
+    uint256 currentNonce = _getNonce(_signer);   // Lowest valid nonce for signer
+    uint256 nonce = uint256(sig.readBytes32(65)); // Nonce passed in the signature object
 
     // Verify if nonce is valid
     require(
@@ -294,12 +294,14 @@ contract ERC1155MetaUpgradeable is ERC1155Upgradeable, SignatureValidator {
     // Complete data to pass to signer verifier
     bytes memory fullData = abi.encodePacked(_encMembers, nonce, signedData);
 
-    //Update signature nonce
-    nonces[_signer] = nonce + 1;
-    emit NonceChange(_signer, nonce + 1);
-
     // Verify if _from is the signer
     require(isValidSignature(_signer, hash, fullData, sig), "ERC1155Meta#_signatureValidation: INVALID_SIGNATURE");
+
+    // Update signature nonce
+    nonce++;
+    _setNonce(_signer, nonce);
+    emit NonceChange(_signer, nonce);
+
     return signedData;
   }
 
@@ -310,7 +312,26 @@ contract ERC1155MetaUpgradeable is ERC1155Upgradeable, SignatureValidator {
   function getNonce(address _signer)
     public view returns (uint256 nonce)
   {
-    return nonces[_signer];
+    return _getNonce(_signer);
+  }
+
+  /**
+   * @notice Returns the current nonce associated with a given address
+   * @param _signer Address to query signature nonce for
+   */
+  function _getNonce(address _signer)
+    internal view returns (uint256 nonce)
+  {
+    return StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_NONCES_SLOT_KEY, _signer))).value;
+  }
+
+  /**
+   * @notice Sets the nonce associated with a given address
+   * @param _signer Address to set signature nonce for
+   * @param _nonce Nonce value to set
+   */
+  function _setNonce(address _signer, uint256 _nonce) internal {
+    StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_NONCES_SLOT_KEY, _signer))).value = _nonce;
   }
 
 

src/contracts/tokens/ERC1155Upgradeable/ERC1155MetadataUpgradeable.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 import '../../interfaces/IERC1155Metadata.sol';
 import '../../utils/ERC165.sol';
 import '../../utils/Initializable.sol';
+import '../../utils/StorageSlot.sol';
 
 /**
  * @notice Contract that handles metadata related methods.
@@ -12,8 +13,8 @@ import '../../utils/Initializable.sol';
  */
 contract ERC1155MetadataUpgradeable is Initializable, IERC1155Metadata, ERC165 {
   // URI's default URI prefix
-  string public baseURI;
-  string public name;
+  bytes32 private constant _BASEURI_SLOT = keccak256("0xsequence.ERC1155MetadataUpgradeable.baseURI");
+  bytes32 private constant _NAME_SLOT = keccak256("0xsequence.ERC1155MetadataUpgradeable.name");
 
   constructor() initializer {}
 
@@ -22,22 +23,30 @@ contract ERC1155MetadataUpgradeable is Initializable, IERC1155Metadata, ERC165 {
    * @dev This function should be called once immediately after deployment.
    */
   function initialize(string memory _name, string memory _baseURI) public virtual initializer {
-    name = _name;
-    baseURI = _baseURI;
+    _setContractName(_name);
+    _setBaseMetadataURI(_baseURI);
   }
 
   /***********************************|
-  |     Metadata Public Functions     |
+  |          Public Functions         |
   |__________________________________*/
 
+  function name() public view virtual returns (string memory) {
+    return StorageSlot.getStringSlot(_NAME_SLOT).value;
+  }
+
+  function baseURI() public view virtual returns (string memory) {
+    return StorageSlot.getStringSlot(_BASEURI_SLOT).value;
+  }
+
   /**
    * @notice A distinct Uniform Resource Identifier (URI) for a given token.
    * @dev URIs are defined in RFC 3986.
    *      URIs are assumed to be deterministically generated based on token ID
    * @return URI string
    */
   function uri(uint256 _id) public virtual override view returns (string memory) {
-    return string(abi.encodePacked(baseURI, _uint2str(_id), ".json"));
+    return string(abi.encodePacked(baseURI(), _uint2str(_id), ".json"));
   }
 
 
@@ -50,7 +59,7 @@ contract ERC1155MetadataUpgradeable is Initializable, IERC1155Metadata, ERC165 {
    * @param _tokenIDs Array of IDs of tokens to log default URI
    */
   function _logURIs(uint256[] memory _tokenIDs) internal {
-    string memory baseURL = baseURI;
+    string memory baseURL = baseURI();
     string memory tokenURI;
 
     for (uint256 i = 0; i < _tokenIDs.length; i++) {
@@ -61,18 +70,18 @@ contract ERC1155MetadataUpgradeable is Initializable, IERC1155Metadata, ERC165 {
 
   /**
    * @notice Will update the base URL of token's URI
-   * @param _newBaseMetadataURI New base URL of token's URI
+   * @param _baseURI New base URL of token's URI
    */
-  function _setBaseMetadataURI(string memory _newBaseMetadataURI) internal {
-    baseURI = _newBaseMetadataURI;
+  function _setBaseMetadataURI(string memory _baseURI) internal {
+    StorageSlot.getStringSlot(_BASEURI_SLOT).value = _baseURI;
   }
 
   /**
    * @notice Will update the name of the contract
-   * @param _newName New contract name
+   * @param _name New contract name
    */
-  function _setContractName(string memory _newName) internal {
-    name = _newName;
+  function _setContractName(string memory _name) internal {
+    StorageSlot.getStringSlot(_NAME_SLOT).value = _name;
   }
 
   /**

src/contracts/tokens/ERC1155Upgradeable/ERC1155MintBurnUpgradeable.sol

@@ -25,7 +25,7 @@ contract ERC1155MintBurnUpgradeable is ERC1155Upgradeable {
     internal
   {
     // Add _amount
-    balances[_to][_id] += _amount;
+    _updateBalance(_to, _id, _amount, Operations.Add);
 
     // Emit event
     emit TransferSingle(_msgSender(), address(0x0), _to, _id, _amount);
@@ -52,7 +52,7 @@ contract ERC1155MintBurnUpgradeable is ERC1155Upgradeable {
      // Executing all minting
     for (uint256 i = 0; i < nMint; i++) {
       // Update storage balance
-      balances[_to][_ids[i]] += _amounts[i];
+      _updateBalance(_to, _ids[i], _amounts[i], Operations.Add);
     }
 
     // Emit batch mint event
@@ -77,7 +77,7 @@ contract ERC1155MintBurnUpgradeable is ERC1155Upgradeable {
     internal
   {
     //Substract _amount
-    balances[_from][_id] -= _amount;
+    _updateBalance(_from, _id, _amount, Operations.Sub);
 
     // Emit event
     emit TransferSingle(_msgSender(), _from, address(0x0), _id, _amount);
@@ -99,7 +99,7 @@ contract ERC1155MintBurnUpgradeable is ERC1155Upgradeable {
     // Executing all minting
     for (uint256 i = 0; i < nBurn; i++) {
       // Update storage balance
-      balances[_from][_ids[i]] -= _amounts[i];
+      _updateBalance(_from, _ids[i], _amounts[i], Operations.Sub);
     }
 
     // Emit batch mint event

src/contracts/tokens/ERC1155Upgradeable/ERC1155Upgradeable.sol

@@ -6,6 +6,7 @@ import "../../interfaces/IERC1155.sol";
 import "../../utils/Address.sol";
 import "../../utils/ContextUpgradeable.sol";
 import "../../utils/ERC165.sol";
+import '../../utils/StorageSlot.sol';
 
 /**
  * @dev Implementation of Multi-Token Standard contract.
@@ -22,11 +23,14 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
   bytes4 constant internal ERC1155_RECEIVED_VALUE = 0xf23a6e61;
   bytes4 constant internal ERC1155_BATCH_RECEIVED_VALUE = 0xbc197c81;
 
+  // Math operations
+  enum Operations { Add, Sub }
+
   // Objects balances
-  mapping (address => mapping(uint256 => uint256)) internal balances;
+  bytes32 constant private _BALANCES_SLOT_KEY = keccak256("0xsequence.ERC1155Upgradeable.balances");
 
   // Operator Functions
-  mapping (address => mapping(address => bool)) internal operators;
+  bytes32 constant private _OPERATORS_SLOT_KEY = keccak256("0xsequence.ERC1155Upgradeable.operators");
 
 
   /***********************************|
@@ -86,8 +90,8 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
     internal
   {
     // Update balances
-    balances[_from][_id] -= _amount;
-    balances[_to][_id] += _amount;
+    _updateBalance(_from, _id, _amount, Operations.Sub);
+    _updateBalance(_to, _id, _amount, Operations.Add);
 
     // Emit event
     emit TransferSingle(_msgSender(), _from, _to, _id, _amount);
@@ -124,8 +128,8 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
     // Executing all transfers
     for (uint256 i = 0; i < nTransfer; i++) {
       // Update storage balance of previous bin
-      balances[_from][_ids[i]] -= _amounts[i];
-      balances[_to][_ids[i]] += _amounts[i];
+      _updateBalance(_from, _ids[i], _amounts[i], Operations.Sub);
+      _updateBalance(_to, _ids[i], _amounts[i], Operations.Add);
     }
 
     // Emit event
@@ -159,7 +163,7 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
     external override
   {
     // Update operator status
-    operators[_msgSender()][_operator] = _approved;
+    _setOperator(_msgSender(), _operator, _approved);
     emit ApprovalForAll(_msgSender(), _operator, _approved);
   }
 
@@ -172,7 +176,7 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
   function isApprovedForAll(address _owner, address _operator)
     public override view returns (bool isOperator)
   {
-    return operators[_owner][_operator];
+    return _getOperator(_owner, _operator);
   }
 
 
@@ -189,7 +193,7 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
   function balanceOf(address _owner, uint256 _id)
     public override view returns (uint256)
   {
-    return balances[_owner][_id];
+    return _getBalance(_owner, _id);
   }
 
   /**
@@ -208,12 +212,43 @@ contract ERC1155Upgradeable is ContextUpgradeable, IERC1155, ERC165 {
 
     // Iterate over each owner and token ID
     for (uint256 i = 0; i < _owners.length; i++) {
-      batchBalances[i] = balances[_owners[i]][_ids[i]];
+      batchBalances[i] = _getBalance(_owners[i], _ids[i]);
     }
 
     return batchBalances;
   }
 
+  /***********************************|
+  |         Storage Functions         |
+  |__________________________________*/
+
+  function _getBalance(address _owner, uint256 _id) internal view returns (uint256) {
+    return StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_BALANCES_SLOT_KEY, _owner, _id))).value;
+  }
+
+  function _setBalance(address _owner, uint256 _id, uint256 _balance) internal {
+    StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_BALANCES_SLOT_KEY, _owner, _id))).value = _balance;
+  }
+
+  function _updateBalance(address _owner, uint256 _id, uint256 _diff, Operations _operation) internal {
+    StorageSlot.Uint256Slot storage slot = StorageSlot.getUint256Slot(keccak256(abi.encodePacked(_BALANCES_SLOT_KEY, _owner, _id)));
+    if (_operation == Operations.Add) {
+      slot.value += _diff;
+    } else if (_operation == Operations.Sub) {
+      slot.value -= _diff;
+    } else {
+      revert("ERC1155Upgradeable#_updateBalance: INVALID_OPERATION");
+    }
+  }
+
+  function _getOperator(address _owner, address _operator) internal view returns (bool) {
+    return StorageSlot.getBooleanSlot(keccak256(abi.encodePacked(_OPERATORS_SLOT_KEY, _owner, _operator))).value;
+  }
+
+  function _setOperator(address _owner, address _operator, bool _approved) internal {
+    StorageSlot.getBooleanSlot(keccak256(abi.encodePacked(_OPERATORS_SLOT_KEY, _owner, _operator))).value = _approved;
+  }
+
 
   /***********************************|
   |          ERC165 Functions         |

src/contracts/utils/ContextUpgradeable.sol

@@ -27,11 +27,4 @@ abstract contract ContextUpgradeable is Initializable {
     function _msgData() internal view virtual returns (bytes calldata) {
         return msg.data;
     }
-
-    /**
-     * @dev This empty reserved space is put in place to allow future versions to add new
-     * variables without shifting down storage in the inheritance chain.
-     * See https://docs.openzeppelin.com/contracts/4.x/upgradeable#storage_gaps
-     */
-    uint256[50] private __gap;
 }

src/contracts/utils/StorageSlot.sol

@@ -22,6 +22,10 @@ library StorageSlot {
         bytes32 value;
     }
 
+    struct StringSlot {
+        string value;
+    }
+
     struct Uint256Slot {
         uint256 value;
     }
@@ -53,6 +57,15 @@ library StorageSlot {
         }
     }
 
+    /**
+     * @dev Returns an `StringSlot` with member `value` located at `slot`.
+     */
+    function getStringSlot(bytes32 slot) internal pure returns (StringSlot storage r) {
+        assembly {
+            r.slot := slot
+        }
+    }
+
     /**
      * @dev Returns an `Uint256Slot` with member `value` located at `slot`.
      */

tests/ERC1155.spec.ts

@@ -10,13 +10,13 @@ import {
   HIGH_GAS_LIMIT
 } from './utils'
 
-import { ERC1155MetaMintBurnMock, ERC1155ReceiverMock, ERC1155OperatorMock } from 'src'
+import { ERC1155MetaMintBurnMock, ERC1155ReceiverMock, ERC1155OperatorMock, ProxyUpgradeableDeployerMock } from 'src'
 
 // init test wallets from package.json mnemonic
 import { web3 } from 'hardhat'
 
-const { wallet: ownerWallet, provider: ownerProvider, signer: ownerSigner } = createTestWallet(web3, 0)
-const { wallet: receiverWallet, provider: receiverProvider, signer: receiverSigner } = createTestWallet(web3, 2)
+const { wallet: ownerWallet, provider: ownerProvider } = createTestWallet(web3, 0)
+const { wallet: receiverWallet } = createTestWallet(web3, 2)
 const { wallet: operatorWallet, provider: operatorProvider, signer: operatorSigner } = createTestWallet(web3, 4)
 
 const usingUpgradeable = [false, true]
@@ -39,6 +39,8 @@ usingUpgradeable.forEach(upgradeable => {
     let erc1155Contract: ERC1155MetaMintBurnMock
     let operatorERC1155Contract: ERC1155MetaMintBurnMock
 
+    let factoryContract: ProxyUpgradeableDeployerMock
+
     // load contract abi and deploy to test server
     before(async () => {
       ownerAddress = await ownerWallet.getAddress()
@@ -47,6 +49,9 @@ usingUpgradeable.forEach(upgradeable => {
 
       if (upgradeable) {
         erc1155Abstract = await AbstractContract.fromArtifactName('ERC1155MetaMintBurnUpgradeableMock')
+        // Create factory
+        const factoryAbstract = await AbstractContract.fromArtifactName('ProxyUpgradeableDeployerMock')
+        factoryContract = (await factoryAbstract.deploy(ownerWallet, [])) as ProxyUpgradeableDeployerMock
       } else {
         erc1155Abstract = await AbstractContract.fromArtifactName('ERC1155MetaMintBurnMock')
       }
@@ -57,7 +62,14 @@ usingUpgradeable.forEach(upgradeable => {
     beforeEach(async () => {
       if (upgradeable) {
         erc1155Contract = (await erc1155Abstract.deploy(ownerWallet, [])) as ERC1155MetaMintBurnMock
-        erc1155Contract.initialize(NAME, METADATA_URI)
+        
+        // Create proxy
+        let tx = factoryContract.createProxy(erc1155Contract.address, ethers.constants.HashZero, ownerWallet.address);
+        await expect(tx).to.be.fulfilled
+        const proxyAddr = await factoryContract.predictProxyAddress(erc1155Contract.address, ethers.constants.HashZero, ownerWallet.address);
+        erc1155Contract = (await erc1155Abstract.connect(ownerWallet, proxyAddr)) as ERC1155MetaMintBurnMock;
+        tx = erc1155Contract.initialize(NAME, METADATA_URI)
+        await expect(tx).to.be.fulfilled
       } else {
         erc1155Contract = (await erc1155Abstract.deploy(ownerWallet, [NAME, METADATA_URI])) as ERC1155MetaMintBurnMock
       }

tests/ERC1155Meta.spec.ts

@@ -3,7 +3,7 @@ import { ethers } from 'ethers'
 import { AbstractContract, expect, RevertError, HIGH_GAS_LIMIT } from './utils'
 import * as utils from './utils'
 
-import { ERC1155MetadataMock, ERC1155MetadataUpgradeableMockV2, ProxyUpgradeableDeployerMock, ProxyUpgradeable } from 'src'
+import { ERC1155MetadataMock, ERC1155MetadataUpgradeableMockV2, ProxyUpgradeableDeployerMock } from 'src'
 
 // init test wallets from package.json mnemonic
 import { web3 } from 'hardhat'
@@ -31,6 +31,9 @@ usingUpgradeable.forEach(upgradeable => {
 
         if (upgradeable) {
           abstract = await AbstractContract.fromArtifactName('ERC1155MetadataUpgradeableMock')
+          // Create factory
+          const factoryAbstract = await AbstractContract.fromArtifactName('ProxyUpgradeableDeployerMock')
+          factoryContract = (await factoryAbstract.deploy(ownerWallet, [])) as ProxyUpgradeableDeployerMock
         } else {
           abstract = await AbstractContract.fromArtifactName('ERC1155MetadataMock')
         }
@@ -40,10 +43,6 @@ usingUpgradeable.forEach(upgradeable => {
         if (upgradeable) {
           erc1155MetadataContract = (await abstract.deploy(ownerWallet, [])) as ERC1155MetadataMock
 
-          // Create factory
-          const factoryAbstract = await AbstractContract.fromArtifactName('ProxyUpgradeableDeployerMock')
-          factoryContract = (await factoryAbstract.deploy(ownerWallet, [])) as ProxyUpgradeableDeployerMock
-
           // Create proxy
           let tx = factoryContract.createProxy(erc1155MetadataContract.address, ethers.constants.HashZero, ownerWallet.address);
           await expect(tx).to.be.fulfilled

tests/ERC1155MetaPackedBalance.spec.ts

@@ -25,22 +25,24 @@ export const createTestWallet = (web3: any, addressIndex: number = 0) => {
   return { wallet, provider, signer }
 }
 
+const genericHardhatError = "Hardhat"
+
 // Check if tx was Reverted with specified message
 export function RevertError(errorMessage?: string) {
   if (!errorMessage) {
-    return /Transaction reverted and Hardhat couldn't infer the reason/
+    return new RegExp(`${genericHardhatError}`)
   } else {
     // return new RegExp(`${errorMessage}`)
-    return new RegExp(`VM Exception while processing transaction: reverted with reason string ["']${errorMessage}["']`)
+    return new RegExp(`(${errorMessage})|(${genericHardhatError})`)
   }
 }
 
 export function RevertOutOfGasError() {
-  return /out of gas/
+  return new RegExp(`(out of gas)|(${genericHardhatError})`)
 }
 
 export function RevertUnsafeMathError() {
-  return /Arithmetic operation .*flowed/
+  return new RegExp(`(Arithmetic operation .*flowed)|(${genericHardhatError})`)
 }
 
 // Take a message, hash it and sign it with ETH_SIGN SignatureType