// SPDX-License-Identifier: MIT // OpenZeppelin Contracts (last updated v4.6.0) (token/ERC721/ERC721.sol)

pragma solidity ^0.8.0;

import "./IERC721.sol"; import "./IERC721Receiver.sol"; import "./extensions/IERC721Metadata.sol"; import "../../utils/Address.sol"; import "../../utils/Context.sol"; import "../../utils/Strings.sol"; import "../../utils/introspection/ERC165.sol";

/**

• @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 { using Address for address; using Strings for uint256;

  // Token name string private _name;

  // Token symbol string private _symbol;

  // Mapping from token ID to owner address mapping(uint256 => address) private _owners;

  // Mapping owner address to token count mapping(address => uint256) private _balances;

  // 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. */ constructor(string memory name_, string memory symbol_) { name = name; symbol = symbol; }

  /**

  • @dev See {IERC165-supportsInterface}. */ function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) { return interfaceId == type(IERC721).interfaceId || interfaceId == type(IERC721Metadata).interfaceId || super.supportsInterface(interfaceId); }

  /**

  • @dev See {IERC721-balanceOf}. */ function balanceOf(address owner) public view virtual override returns (uint256) { require(owner != address(0), "ERC721: balance query for the zero address"); return _balances[owner]; }

  /**

  • @dev See {IERC721-ownerOf}. */ function ownerOf(uint256 tokenId) public view virtual override returns (address) { address owner = _owners[tokenId]; require(owner != address(0), "ERC721: owner query for nonexistent token"); return owner; }

  /**

  • @dev See {IERC721Metadata-name}. */ function name() public view virtual override returns (string memory) { return _name; }

  /**

  • @dev See {IERC721Metadata-symbol}. */ function symbol() public view virtual override returns (string memory) { return _symbol; }

  /**

  • @dev See {IERC721Metadata-tokenURI}. */ function tokenURI(uint256 tokenId) public view virtual override returns (string memory) { require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

    string memory baseURI = _baseURI(); return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : ""; }

  /**

  • @dev Base URI for computing {tokenURI}. If set, the resulting URI for each

  • token will be the concatenation of the baseURI and the tokenId. Empty

  • by default, can be overridden in child contracts. */ function _baseURI() internal view virtual returns (string memory) { return ""; }

  /**

  • @dev See {IERC721-approve}. */ function approve(address to, uint256 tokenId) public virtual override { address owner = ERC721.ownerOf(tokenId); require(to != owner, "ERC721: approval to current owner");

    require( _msgSender() == owner || isApprovedForAll(owner, _msgSender()), "ERC721: approve caller is not owner nor approved for all" );

    _approve(to, tokenId); }

  /**

  • @dev See {IERC721-getApproved}. */ function getApproved(uint256 tokenId) public view virtual override returns (address) { require(_exists(tokenId), "ERC721: approved query for nonexistent token");

    return _tokenApprovals[tokenId]; }

  /**

  • @dev See {IERC721-setApprovalForAll}. */ function setApprovalForAll(address operator, bool approved) public virtual override { _setApprovalForAll(_msgSender(), operator, approved); }

  /**

  • @dev See {IERC721-isApprovedForAll}. */ function isApprovedForAll(address owner, address operator) public view virtual override returns (bool) { return _operatorApprovals[owner][operator]; }

  /**

  • @dev See {IERC721-transferFrom}. */ function transferFrom( address from, address to, uint256 tokenId ) public virtual override { //solhint-disable-next-line max-line-length require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

    _transfer(from, to, tokenId); }

  /**

  • @dev See {IERC721-safeTransferFrom}. */ function safeTransferFrom( address from, address to, uint256 tokenId ) public virtual override { safeTransferFrom(from, to, tokenId, ""); }

  /**

  • @dev See {IERC721-safeTransferFrom}. */ function safeTransferFrom( address from, address to, uint256 tokenId, bytes memory _data ) public virtual override { require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved"); _safeTransfer(from, to, tokenId, _data); }

  /**

  • @dev Safely transfers tokenId token from from to to, checking first that contract recipients

  • are aware of the ERC721 protocol to prevent tokens from being forever locked.

  • _data is additional data, it has no specified format and it is sent in call to to.

  • This internal function is equivalent to {safeTransferFrom}, and can be used to e.g.

  • implement alternative mechanisms to perform token transfer, such as signature-based.

  • Requirements:

  • from cannot be the zero address.

  • to cannot be the zero address.

  • tokenId token must exist and be owned by from.

  • If to refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.

  • Emits a {Transfer} event. */ function _safeTransfer( address from, address to, uint256 tokenId, bytes memory _data ) internal virtual { _transfer(from, to, tokenId); require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer"); }

  /**

  • @dev Returns whether tokenId exists.

  • Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.

  • Tokens start existing when they are minted (_mint),

  • and stop existing when they are burned (_burn). */ function _exists(uint256 tokenId) internal view virtual returns (bool) { return _owners[tokenId] != address(0); }

  /**

  • @dev Returns whether spender is allowed to manage tokenId.

  • Requirements:

  • tokenId must exist. */ function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) { require(_exists(tokenId), "ERC721: operator query for nonexistent token"); address owner = ERC721.ownerOf(tokenId); return (spender == owner || isApprovedForAll(owner, spender) || getApproved(tokenId) == spender); }

  /**

  • @dev Safely mints tokenId and transfers it to to.

  • Requirements:

  • tokenId must not exist.

  • If to refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.

  • Emits a {Transfer} event. */ function _safeMint(address to, uint256 tokenId) internal virtual { _safeMint(to, tokenId, ""); }

  /**

  • @dev Same as {xref-ERC721-_safeMint-address-uint256-}[_safeMint], with an additional data parameter which is

  • forwarded in {IERC721Receiver-onERC721Received} to contract recipients. */ 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" ); }

  /**

  • @dev Mints tokenId and transfers it to to.

  • WARNING: Usage of this method is discouraged, use {_safeMint} whenever possible

  • Requirements:

  • tokenId must not exist.

  • to cannot be the zero address.

  • Emits a {Transfer} event. */ function _mint(address to, uint256 tokenId) internal virtual { require(to != address(0), "ERC721: mint to the zero address"); require(!_exists(tokenId), "ERC721: token already minted");

    _beforeTokenTransfer(address(0), to, tokenId);

    _balances[to] += 1; _owners[tokenId] = to;

    emit Transfer(address(0), to, tokenId);

    _afterTokenTransfer(address(0), to, tokenId); }

  /**

  • @dev Destroys tokenId.

  • The approval is cleared when the token is burned.

  • Requirements:

  • tokenId must exist.

  • Emits a {Transfer} event. */ function _burn(uint256 tokenId) internal virtual { address owner = ERC721.ownerOf(tokenId);

    _beforeTokenTransfer(owner, address(0), tokenId);

    // Clear approvals _approve(address(0), tokenId);

    _balances[owner] -= 1; delete _owners[tokenId];

    emit Transfer(owner, address(0), tokenId);

    _afterTokenTransfer(owner, address(0), tokenId); }

  /**

  • @dev Transfers tokenId from from to to.

  • As opposed to {transferFrom}, this imposes no restrictions on msg.sender.

  • Requirements:

  • to cannot be the zero address.

  • tokenId token must be owned by from.

  • Emits a {Transfer} event. */ function _transfer( address from, address to, uint256 tokenId ) internal virtual { require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner"); require(to != address(0), "ERC721: transfer to the zero address");

    _beforeTokenTransfer(from, to, tokenId);

    // Clear approvals from the previous owner _approve(address(0), tokenId);

    _balances[from] -= 1; _balances[to] += 1; _owners[tokenId] = to;

    emit Transfer(from, to, tokenId);

    _afterTokenTransfer(from, to, tokenId); }

  /**

  • @dev Approve to to operate on tokenId

  • Emits a {Approval} event. */ function _approve(address to, uint256 tokenId) internal virtual { _tokenApprovals[tokenId] = to; emit Approval(ERC721.ownerOf(tokenId), to, tokenId); }

  /**

  • @dev Approve operator to operate on all of owner tokens

  • Emits a {ApprovalForAll} event. */ function _setApprovalForAll( address owner, address operator, bool approved ) internal virtual { require(owner != operator, "ERC721: approve to caller"); _operatorApprovals[owner][operator] = approved; emit ApprovalForAll(owner, operator, approved); }

  /**

  • @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.

  • The call is not executed if the target address is not a contract.

  • @param from address representing the previous owner of the given token ID

  • @param to target address that will receive the tokens

  • @param tokenId uint256 ID of the token to be transferred

  • @param _data bytes optional data to send along with the call

  • @return bool whether the call correctly returned the expected magic value */ function _checkOnERC721Received( address from, address to, uint256 tokenId, bytes memory _data ) private returns (bool) { if (to.isContract()) { try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) { return retval == IERC721Receiver.onERC721Received.selector; } catch (bytes memory reason) { if (reason.length == 0) { revert("ERC721: transfer to non ERC721Receiver implementer"); } else { assembly { revert(add(32, reason), mload(reason)) } } } } else { return true; } }

  /**

  • @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 and to are never both zero.

  • 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 {}

  /**

  • @dev Hook that is called after any transfer of tokens. This includes

  • minting and burning.

  • Calling conditions:

  • when from and to are both non-zero.

  • from and to are never both zero.

  • To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks]. */ function _afterTokenTransfer( address from, address to, uint256 tokenId ) internal virtual {} }