# ERC 721 Codes

**Published by:** [journeywest.eth](https://paragraph.com/@journeywest-eth/)
**Published on:** 2022-06-09
**URL:** https://paragraph.com/@journeywest-eth/erc-721-codes

## Content

// 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 {} }

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