Remove enumerable from ERC721 and add an ERC721Enumerable extension (#2511)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

CHANGELOG.md

@@ -10,6 +10,7 @@
  * `GSN`: Deprecate GSNv1 support in favor of upcomming support for GSNv2. ([#2521](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2521))
  * `ERC165`: Remove uses of storage in the base ERC165 implementation. ERC165 based contracts now use storage-less virtual functions. Old behaviour remains available in the `ERC165Storage` extension. ([#2505](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2505))
  * `Initializable`: Make initializer check stricter during construction. ([#2531](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2531))
+ * `ERC721`: remove enumerability of tokens from the base implementation. This feature is now provided separately through the `ERC721Enumerable` extension. ([#2511](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2511))
 
 ## 3.4.0 (2021-02-02)
 

contracts/mocks/ERC721EnumerableMock.sol

+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../token/ERC721/ERC721Enumerable.sol";
+
+/**
+ * @title ERC721Mock
+ * This mock just provides a public safeMint, mint, and burn functions for testing purposes
+ */
+contract ERC721EnumerableMock is ERC721Enumerable {
+    string private _baseTokenURI;
+
+    constructor (string memory name, string memory symbol) ERC721(name, symbol) { }
+
+    function _baseURI() internal view virtual override returns (string memory) {
+        return _baseTokenURI;
+    }
+
+    function setBaseURI(string calldata newBaseTokenURI) public {
+        _baseTokenURI = newBaseTokenURI;
+    }
+
+    function baseURI() public view returns (string memory) {
+        return _baseURI();
+    }
+
+    function mint(address to, uint256 tokenId) public {
+        _mint(to, tokenId);
+    }
+
+    function safeMint(address to, uint256 tokenId) public {
+        _safeMint(to, tokenId);
+    }
+
+    function safeMint(address to, uint256 tokenId, bytes memory _data) public {
+        _safeMint(to, tokenId, _data);
+    }
+
+    function burn(uint256 tokenId) public {
+        _burn(tokenId);
+    }
+}

contracts/mocks/ERC721Mock.sol

@@ -9,18 +9,24 @@ import "../token/ERC721/ERC721.sol";
  * This mock just provides a public safeMint, mint, and burn functions for testing purposes
  */
 contract ERC721Mock is ERC721 {
+    string private _baseTokenURI;
+
     constructor (string memory name, string memory symbol) ERC721(name, symbol) { }
 
-    function exists(uint256 tokenId) public view returns (bool) {
-        return _exists(tokenId);
+    function _baseURI() internal view virtual override returns (string memory) {
+        return _baseTokenURI;
     }
 
-    function setTokenURI(uint256 tokenId, string memory uri) public {
-        _setTokenURI(tokenId, uri);
+    function setBaseURI(string calldata newBaseTokenURI) public {
+        _baseTokenURI = newBaseTokenURI;
     }
 
-    function setBaseURI(string memory baseURI) public {
-        _setBaseURI(baseURI);
+    function baseURI() public view returns (string memory) {
+        return _baseURI();
+    }
+
+    function exists(uint256 tokenId) public view returns (bool) {
+        return _exists(tokenId);
     }
 
     function mint(address to, uint256 tokenId) public {

contracts/presets/ERC721PresetMinterPauserAutoId.sol

@@ -6,6 +6,7 @@ import "../access/AccessControl.sol";
 import "../utils/Context.sol";
 import "../utils/Counters.sol";
 import "../token/ERC721/ERC721.sol";
+import "../token/ERC721/ERC721Enumerable.sol";
 import "../token/ERC721/ERC721Burnable.sol";
 import "../token/ERC721/ERC721Pausable.sol";
 
@@ -24,7 +25,7 @@ import "../token/ERC721/ERC721Pausable.sol";
  * roles, as well as the default admin role, which will let it grant both minter
  * and pauser roles to other accounts.
  */
-contract ERC721PresetMinterPauserAutoId is Context, AccessControl, ERC721Burnable, ERC721Pausable {
+contract ERC721PresetMinterPauserAutoId is Context, AccessControl, ERC721Enumerable, ERC721Burnable, ERC721Pausable {
     using Counters for Counters.Counter;
 
     bytes32 public constant MINTER_ROLE = keccak256("MINTER_ROLE");
@@ -32,6 +33,8 @@ contract ERC721PresetMinterPauserAutoId is Context, AccessControl, ERC721Burnabl
 
     Counters.Counter private _tokenIdTracker;
 
+    string private _baseTokenURI;
+
     /**
      * @dev Grants `DEFAULT_ADMIN_ROLE`, `MINTER_ROLE` and `PAUSER_ROLE` to the
      * account that deploys the contract.
@@ -39,13 +42,17 @@ contract ERC721PresetMinterPauserAutoId is Context, AccessControl, ERC721Burnabl
      * Token URIs will be autogenerated based on `baseURI` and their token IDs.
      * See {ERC721-tokenURI}.
      */
-    constructor(string memory name, string memory symbol, string memory baseURI) ERC721(name, symbol) {
+    constructor(string memory name, string memory symbol, string memory baseTokenURI) ERC721(name, symbol) {
+        _baseTokenURI = baseTokenURI;
+
         _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
 
         _setupRole(MINTER_ROLE, _msgSender());
         _setupRole(PAUSER_ROLE, _msgSender());
+    }
 
-        _setBaseURI(baseURI);
+    function _baseURI() internal view virtual override returns (string memory) {
+        return _baseTokenURI;
     }
 
     /**
@@ -96,7 +103,14 @@ contract ERC721PresetMinterPauserAutoId is Context, AccessControl, ERC721Burnabl
         _unpause();
     }
 
-    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual override(ERC721, ERC721Pausable) {
+    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual override(ERC721, ERC721Enumerable, ERC721Pausable) {
         super._beforeTokenTransfer(from, to, tokenId);
     }
+
+    /**
+     * @dev See {IERC165-supportsInterface}.
+     */
+    function supportsInterface(bytes4 interfaceId) public view virtual override(ERC721, ERC721Enumerable) returns (bool) {
+        return super.supportsInterface(interfaceId);
+    }
 }

contracts/token/ERC721/ERC721.sol

@@ -5,47 +5,37 @@ pragma solidity ^0.8.0;
 import "../../utils/Context.sol";
 import "./IERC721.sol";
 import "./IERC721Metadata.sol";
-import "./IERC721Enumerable.sol";
 import "./IERC721Receiver.sol";
 import "../../introspection/ERC165.sol";
 import "../../utils/Address.sol";
-import "../../utils/EnumerableSet.sol";
-import "../../utils/EnumerableMap.sol";
 import "../../utils/Strings.sol";
 
 /**
- * @title ERC721 Non-Fungible Token Standard basic implementation
- * @dev see https://eips.ethereum.org/EIPS/eip-721
+ * @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including
+ * the Metadata extension, but not including the Enumerable extension, which is available separately as
+ * {ERC721Enumerable}.
  */
-contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable {
+contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
     using Address for address;
-    using EnumerableSet for EnumerableSet.UintSet;
-    using EnumerableMap for EnumerableMap.UintToAddressMap;
     using Strings for uint256;
 
-    // Mapping from holder address to their (enumerable) set of owned tokens
-    mapping (address => EnumerableSet.UintSet) private _holderTokens;
-
-    // Enumerable mapping from token ids to their owners
-    EnumerableMap.UintToAddressMap private _tokenOwners;
-
-    // Mapping from token ID to approved address
-    mapping (uint256 => address) private _tokenApprovals;
-
-    // Mapping from owner to operator approvals
-    mapping (address => mapping (address => bool)) private _operatorApprovals;
-
     // Token name
     string private _name;
 
     // Token symbol
     string private _symbol;
 
-    // Optional mapping for token URIs
-    mapping (uint256 => string) private _tokenURIs;
+    // Mapping from token ID to owner address
+    mapping (uint256 => address) private _owners;
+
+    // Mapping owner address to token count
+    mapping (address => uint256) private _balances;
 
-    // Base URI
-    string private _baseURI;
+    // Mapping from token ID to approved address
+    mapping (uint256 => address) private _tokenApprovals;
+
+    // Mapping from owner to operator approvals
+    mapping (address => mapping (address => bool)) private _operatorApprovals;
 
     /**
      * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
@@ -61,7 +51,6 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
     function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) {
         return interfaceId == type(IERC721).interfaceId
             || interfaceId == type(IERC721Metadata).interfaceId
-            || interfaceId == type(IERC721Enumerable).interfaceId
             || super.supportsInterface(interfaceId);
     }
 
@@ -70,14 +59,16 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
      */
     function balanceOf(address owner) public view virtual override returns (uint256) {
         require(owner != address(0), "ERC721: balance query for the zero address");
-        return _holderTokens[owner].length();
+        return _balances[owner];
     }
 
     /**
      * @dev See {IERC721-ownerOf}.
      */
     function ownerOf(uint256 tokenId) public view virtual override returns (address) {
-        return _tokenOwners.get(tokenId, "ERC721: owner query for nonexistent token");
+        address owner = _owners[tokenId];
+        require(owner != address(0), "ERC721: owner query for nonexistent token");
+        return owner;
     }
 
     /**
@@ -100,51 +91,18 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
     function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
         require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");
 
-        string memory _tokenURI = _tokenURIs[tokenId];
-        string memory base = baseURI();
-
-        // If there is no base URI, return the token URI.
-        if (bytes(base).length == 0) {
-            return _tokenURI;
-        }
-        // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
-        if (bytes(_tokenURI).length > 0) {
-            return string(abi.encodePacked(base, _tokenURI));
-        }
-        // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
-        return string(abi.encodePacked(base, tokenId.toString()));
-    }
-
-    /**
-    * @dev Returns the base URI set via {_setBaseURI}. This will be
-    * automatically added as a prefix in {tokenURI} to each token's URI, or
-    * to the token ID if no specific URI is set for that token ID.
-    */
-    function baseURI() public view virtual returns (string memory) {
-        return _baseURI;
-    }
-
-    /**
-     * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
-     */
-    function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual override returns (uint256) {
-        return _holderTokens[owner].at(index);
-    }
-
-    /**
-     * @dev See {IERC721Enumerable-totalSupply}.
-     */
-    function totalSupply() public view virtual override returns (uint256) {
-        // _tokenOwners are indexed by tokenIds, so .length() returns the number of tokenIds
-        return _tokenOwners.length();
+        string memory baseURI = _baseURI();
+        return bytes(baseURI).length > 0
+            ? string(abi.encodePacked(baseURI, tokenId.toString()))
+            : '';
     }
 
     /**
-     * @dev See {IERC721Enumerable-tokenByIndex}.
+     * @dev Base URI for computing {tokenURI}. Empty by default, can be overriden
+     * in child contracts.
      */
-    function tokenByIndex(uint256 index) public view virtual override returns (uint256) {
-        (uint256 tokenId, ) = _tokenOwners.at(index);
-        return tokenId;
+    function _baseURI() internal view virtual returns (string memory) {
+        return "";
     }
 
     /**
@@ -244,7 +202,7 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
      * and stop existing when they are burned (`_burn`).
      */
     function _exists(uint256 tokenId) internal view virtual returns (bool) {
-        return _tokenOwners.contains(tokenId);
+        return _owners[tokenId] != address(0);
     }
 
     /**
@@ -301,9 +259,8 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
 
         _beforeTokenTransfer(address(0), to, tokenId);
 
-        _holderTokens[to].add(tokenId);
-
-        _tokenOwners.set(tokenId, to);
+        _balances[to] += 1;
+        _owners[tokenId] = to;
 
         emit Transfer(address(0), to, tokenId);
     }
@@ -319,21 +276,15 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
      * Emits a {Transfer} event.
      */
     function _burn(uint256 tokenId) internal virtual {
-        address owner = ERC721.ownerOf(tokenId); // internal owner
+        address owner = ERC721.ownerOf(tokenId);
 
         _beforeTokenTransfer(owner, address(0), tokenId);
 
         // Clear approvals
         _approve(address(0), tokenId);
 
-        // Clear metadata (if any)
-        if (bytes(_tokenURIs[tokenId]).length != 0) {
-            delete _tokenURIs[tokenId];
-        }
-
-        _holderTokens[owner].remove(tokenId);
-
-        _tokenOwners.remove(tokenId);
+        _balances[owner] -= 1;
+        delete _owners[tokenId];
 
         emit Transfer(owner, address(0), tokenId);
     }
@@ -350,7 +301,7 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
      * Emits a {Transfer} event.
      */
     function _transfer(address from, address to, uint256 tokenId) internal virtual {
-        require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer of token that is not own"); // internal owner
+        require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
         require(to != address(0), "ERC721: transfer to the zero address");
 
         _beforeTokenTransfer(from, to, tokenId);
@@ -358,33 +309,21 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
         // Clear approvals from the previous owner
         _approve(address(0), tokenId);
 
-        _holderTokens[from].remove(tokenId);
-        _holderTokens[to].add(tokenId);
-
-        _tokenOwners.set(tokenId, to);
+        _balances[from] -= 1;
+        _balances[to] += 1;
+        _owners[tokenId] = to;
 
         emit Transfer(from, to, tokenId);
     }
 
     /**
-     * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
+     * @dev Approve `to` to operate on `tokenId`
      *
-     * Requirements:
-     *
-     * - `tokenId` must exist.
+     * Emits a {Approval} event.
      */
-    function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
-        require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
-        _tokenURIs[tokenId] = _tokenURI;
-    }
-
-    /**
-     * @dev Internal function to set the base URI for all token IDs. It is
-     * automatically added as a prefix to the value returned in {tokenURI},
-     * or to the token ID if {tokenURI} is empty.
-     */
-    function _setBaseURI(string memory baseURI_) internal virtual {
-        _baseURI = baseURI_;
+    function _approve(address to, uint256 tokenId) internal virtual {
+        _tokenApprovals[tokenId] = to;
+        emit Approval(ERC721.ownerOf(tokenId), to, tokenId);
     }
 
     /**
@@ -418,11 +357,6 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable
         }
     }
 
-    function _approve(address to, uint256 tokenId) private {
-        _tokenApprovals[tokenId] = to;
-        emit Approval(ERC721.ownerOf(tokenId), to, tokenId); // internal owner
-    }
-
     /**
      * @dev Hook that is called before any token transfer. This includes minting
      * and burning.

contracts/token/ERC721/ERC721Enumerable.sol

+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "./ERC721.sol";
+import "./IERC721Enumerable.sol";
+
+/**
+ * @dev This implements an optional extension of {ERC721} defined in the EIP that adds
+ * enumerability of all the token ids in the contract as well as all token ids owned by each
+ * account.
+ */
+abstract contract ERC721Enumerable is ERC721, IERC721Enumerable {
+    // Mapping from owner to list of owned token IDs
+    mapping(address => mapping(uint256 => uint256)) private _ownedTokens;
+
+    // Mapping from token ID to index of the owner tokens list
+    mapping(uint256 => uint256) private _ownedTokensIndex;
+
+    // Array with all token ids, used for enumeration
+    uint256[] private _allTokens;
+
+    // Mapping from token id to position in the allTokens array
+    mapping(uint256 => uint256) private _allTokensIndex;
+
+    /**
+     * @dev See {IERC165-supportsInterface}.
+     */
+    function supportsInterface(bytes4 interfaceId) public view virtual override(IERC165, ERC721) returns (bool) {
+        return interfaceId == type(IERC721Enumerable).interfaceId
+            || super.supportsInterface(interfaceId);
+    }
+
+    /**
+     * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
+     */
+    function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual override returns (uint256) {
+        require(index < ERC721.balanceOf(owner), "ERC721Enumerable: owner index out of bounds");
+        return _ownedTokens[owner][index];
+    }
+
+    /**
+     * @dev See {IERC721Enumerable-totalSupply}.
+     */
+    function totalSupply() public view virtual override returns (uint256) {
+        return _allTokens.length;
+    }
+
+    /**
+     * @dev See {IERC721Enumerable-tokenByIndex}.
+     */
+    function tokenByIndex(uint256 index) public view virtual override returns (uint256) {
+        require(index < ERC721Enumerable.totalSupply(), "ERC721Enumerable: global index out of bounds");
+        return _allTokens[index];
+    }
+
+    /**
+     * @dev Hook that is called before any token transfer. This includes minting
+     * and burning.
+     *
+     * Calling conditions:
+     *
+     * - When `from` and `to` are both non-zero, ``from``'s `tokenId` will be
+     * transferred to `to`.
+     * - When `from` is zero, `tokenId` will be minted for `to`.
+     * - When `to` is zero, ``from``'s `tokenId` will be burned.
+     * - `from` cannot be the zero address.
+     * - `to` cannot be the zero address.
+     *
+     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
+     */
+    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual override {
+        super._beforeTokenTransfer(from, to, tokenId);
+
+        if (from == address(0)) {
+            _addTokenToAllTokensEnumeration(tokenId);
+        } else if (from != to) {
+            _removeTokenFromOwnerEnumeration(from, tokenId);
+        }
+        if (to == address(0)) {
+            _removeTokenFromAllTokensEnumeration(tokenId);
+        } else if (to != from) {
+            _addTokenToOwnerEnumeration(to, tokenId);
+        }
+    }
+
+    /**
+     * @dev Private function to add a token to this extension's ownership-tracking data structures.
+     * @param to address representing the new owner of the given token ID
+     * @param tokenId uint256 ID of the token to be added to the tokens list of the given address
+     */
+    function _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
+        uint256 length = ERC721.balanceOf(to);
+        _ownedTokens[to][length] = tokenId;
+        _ownedTokensIndex[tokenId] = length;
+    }
+
+    /**
+     * @dev Private function to add a token to this extension's token tracking data structures.
+     * @param tokenId uint256 ID of the token to be added to the tokens list
+     */
+    function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
+        _allTokensIndex[tokenId] = _allTokens.length;
+        _allTokens.push(tokenId);
+    }
+
+    /**
+     * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
+     * while the token is not assigned a new owner, the `_ownedTokensIndex` mapping is _not_ updated: this allows for
+     * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
+     * This has O(1) time complexity, but alters the order of the _ownedTokens array.
+     * @param from address representing the previous owner of the given token ID
+     * @param tokenId uint256 ID of the token to be removed from the tokens list of the given address
+     */
+    function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
+        // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
+        // then delete the last slot (swap and pop).
+
+        uint256 lastTokenIndex = ERC721.balanceOf(from) - 1;
+        uint256 tokenIndex = _ownedTokensIndex[tokenId];
+
+        // When the token to delete is the last token, the swap operation is unnecessary
+        if (tokenIndex != lastTokenIndex) {
+            uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];
+
+            _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
+            _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
+        }
+
+        // This also deletes the contents at the last position of the array
+        delete _ownedTokensIndex[tokenId];
+        delete _ownedTokens[from][lastTokenIndex];
+    }
+
+    /**
+     * @dev Private function to remove a token from this extension's token tracking data structures.
+     * This has O(1) time complexity, but alters the order of the _allTokens array.
+     * @param tokenId uint256 ID of the token to be removed from the tokens list
+     */
+    function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
+        // To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
+        // then delete the last slot (swap and pop).
+
+        uint256 lastTokenIndex = _allTokens.length - 1;
+        uint256 tokenIndex = _allTokensIndex[tokenId];
+
+        // When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
+        // rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
+        // an 'if' statement (like in _removeTokenFromOwnerEnumeration)
+        uint256 lastTokenId = _allTokens[lastTokenIndex];
+
+        _allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
+        _allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
+
+        // This also deletes the contents at the last position of the array
+        delete _allTokensIndex[tokenId];
+        _allTokens.pop();
+    }
+}

contracts/token/ERC721/README.adoc

@@ -7,7 +7,7 @@ This set of interfaces, contracts, and utilities are all related to the https://
 
 TIP: For a walk through on how to create an ERC721 token read our xref:ROOT:erc721.adoc[ERC721 guide].
 
-The EIP consists of three interfaces, found here as {IERC721}, {IERC721Metadata}, and {IERC721Enumerable}. Only the first one is required in a contract to be ERC721 compliant. However, all three are implemented in {ERC721}.
+The EIP consists of three interfaces, found here as {IERC721}, {IERC721Metadata}, and {IERC721Enumerable}. Only the first one is required in a contract to be ERC721 compliant. The core interface and the metadata extension are both implemented in {ERC721}. The enumerable extension is provided separately in {ERC721Enumerable}.
 
 Additionally, {IERC721Receiver} can be used to prevent tokens from becoming forever locked in contracts. Imagine sending an in-game item to an exchange address that can't send it back!. When using <<IERC721-safeTransferFrom,`safeTransferFrom`>>, the token contract checks to see that the receiver is an {IERC721Receiver}, which implies that it knows how to handle {ERC721} tokens. If you're writing a contract that needs to receive {ERC721} tokens, you'll want to include this interface.
 
@@ -29,6 +29,8 @@ NOTE: This core set of contracts is designed to be unopinionated, allowing devel
 
 {{ERC721}}
 
+{{ERC721Enumerable}}
+
 {{IERC721Receiver}}
 
 == Extensions

test/presets/ERC721PresetMinterPauserAutoId.test.js

@@ -27,10 +27,6 @@ contract('ERC721PresetMinterPauserAutoId', function (accounts) {
     expect(await this.token.symbol()).to.equal(symbol);
   });
 
-  it('token has correct base URI', async function () {
-    expect(await this.token.baseURI()).to.equal(baseURI);
-  });
-
   it('deployer has the default admin role', async function () {
     expect(await this.token.getRoleMemberCount(DEFAULT_ADMIN_ROLE)).to.be.bignumber.equal('1');
     expect(await this.token.getRoleMember(DEFAULT_ADMIN_ROLE, 0)).to.equal(deployer);

test/token/ERC721/ERC721Enumerable.test.js

+const {
+  shouldBehaveLikeERC721,
+  shouldBehaveLikeERC721Metadata,
+  shouldBehaveLikeERC721Enumerable,
+} = require('./ERC721.behavior');
+
+const ERC721Mock = artifacts.require('ERC721EnumerableMock');
+
+contract('ERC721Enumerable', function (accounts) {
+  const name = 'Non Fungible Token';
+  const symbol = 'NFT';
+
+  beforeEach(async function () {
+    this.token = await ERC721Mock.new(name, symbol);
+  });
+
+  shouldBehaveLikeERC721('ERC721', ...accounts);
+  shouldBehaveLikeERC721Metadata('ERC721', name, symbol, ...accounts);
+  shouldBehaveLikeERC721Enumerable('ERC721', ...accounts);
+});