# OpenZeppelin ERC721 ## Intro {% embed url="" %} OpenZeppelin ERC721 implementation {% endembed %} Important functions: * `setApprovalForAll()` * `transferFrom()` * `safeTransferFrom()` ## setApprovalForAll ```solidity /** * @dev See {IERC721-setApprovalForAll}. */ function setApprovalForAll(address operator, bool approved) public virtual override { _setApprovalForAll(_msgSender(), operator, approved); } ``` ```solidity /** * @dev Approve `operator` to operate on all of `owner` tokens * * Emits an {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); } ``` This function is extremely dangerous: **it approves an "operator" to handle all of your NFTs**. You can imagine that it is hazardous in a phishing scenario. ## transferFrom ```solidity /** * @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: caller is not token owner or approved"); _transfer(from, to, tokenId); } ``` ```solidity /** * @dev Returns whether `spender` is allowed to manage `tokenId`. * * Requirements: * * - `tokenId` must exist. */ function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) { address owner = ERC721.ownerOf(tokenId); return (spender == owner || isApprovedForAll(owner, spender) || getApproved(tokenId) == spender); } ``` ```solidity /** * @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, 1); // Check that tokenId was not transferred by `_beforeTokenTransfer` hook require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner"); // Clear approvals from the previous owner delete _tokenApprovals[tokenId]; unchecked { // `_balances[from]` cannot overflow for the same reason as described in `_burn`: // `from`'s balance is the number of token held, which is at least one before the current // transfer. // `_balances[to]` could overflow in the conditions described in `_mint`. That would require // all 2**256 token ids to be minted, which in practice is impossible. _balances[from] -= 1; _balances[to] += 1; } _owners[tokenId] = to; emit Transfer(from, to, tokenId); _afterTokenTransfer(from, to, tokenId, 1); } ``` **There is no transfer function in ERC721 (only** `_transfer()` **exists).** There are 3 cases where you can call `transferFrom()`: 1. You are the owner. 2. You are the operator set by `setApprovalForAll()`. 3. You are approved to handle this `tokenId`. ## safeTransferFrom ```solidity /** * @dev See {IERC721-safeTransferFrom}. */ function safeTransferFrom(address from, address to, uint256 tokenId) public virtual override { safeTransferFrom(from, to, tokenId, ""); } ``` ```solidity /** * @dev See {IERC721-safeTransferFrom}. */ function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public virtual override { require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: caller is not token owner or approved"); _safeTransfer(from, to, tokenId, data); } ``` ```solidity /** * @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"); } ``` ```solidity /** * @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 { /// @solidity memory-safe-assembly assembly { revert(add(32, reason), mload(reason)) } } } } else { return true; } } ``` The receiving contract must implement `onERC721Received()`. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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