# ERC 721 Codes

By [journeywest.eth](https://paragraph.com/@journeywest-eth) · 2022-06-09

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// 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\[ERC721\]](https://eips.ethereum.org/EIPS/eip-721%5BERC721%5D) 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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*Originally published on [journeywest.eth](https://paragraph.com/@journeywest-eth/erc-721-codes)*