ERC 721

This document is better viewed at https://docs.openzeppelin.com/contracts/api/token/erc721

This set of interfaces, contracts, and utilities are all related to the ERC721 Non-Fungible Token Standard.

For a walk through on how to create an ERC721 token read our 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. 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 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.

Additionally there are multiple custom extensions, including:

This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC721 (such as _mint) and expose them as external functions in the way they prefer. On the other hand, ERC721 Presets (such as ERC721PresetMinterPauserAutoId) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.

Core

IERC721 

import "@openzeppelin/contracts/token/ERC721/IERC721.sol";

Required interface of an ERC721 compliant contract.

Functions
IERC165
Events

balanceOf(address owner) → uint256 balance external

Returns the number of tokens in owner's account.

ownerOf(uint256 tokenId) → address owner external

Returns the owner of the tokenId token.

Requirements:

  • tokenId must exist.

safeTransferFrom(address from, address to, uint256 tokenId) external

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.

Requirements:

  • from cannot be the zero address.

  • to cannot be the zero address.

  • tokenId token must exist and be owned by from.

  • If the caller is not from, it must be have been allowed to move this token by either approve or setApprovalForAll.

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

Emits a Transfer event.

transferFrom(address from, address to, uint256 tokenId) external

Transfers tokenId token from from to to.

Usage of this method is discouraged, use safeTransferFrom whenever possible.

Requirements:

  • from cannot be the zero address.

  • to cannot be the zero address.

  • tokenId token must be owned by from.

  • If the caller is not from, it must be approved to move this token by either approve or setApprovalForAll.

Emits a Transfer event.

approve(address to, uint256 tokenId) external

Gives permission to to to transfer tokenId token to another account. The approval is cleared when the token is transferred.

Only a single account can be approved at a time, so approving the zero address clears previous approvals.

Requirements:

  • The caller must own the token or be an approved operator.

  • tokenId must exist.

Emits an Approval event.

getApproved(uint256 tokenId) → address operator external

Returns the account approved for tokenId token.

Requirements:

  • tokenId must exist.

setApprovalForAll(address operator, bool _approved) external

Approve or remove operator as an operator for the caller. Operators can call transferFrom or safeTransferFrom for any token owned by the caller.

Requirements:

  • The operator cannot be the caller.

Emits an ApprovalForAll event.

isApprovedForAll(address owner, address operator) → bool external

Returns if the operator is allowed to manage all of the assets of owner.

See setApprovalForAll

safeTransferFrom(address from, address to, uint256 tokenId, bytes data) external

Safely transfers tokenId token from from to to.

Requirements:

  • from cannot be the zero address.

  • to cannot be the zero address.

  • tokenId token must exist and be owned by from.

  • If the caller is not from, it must be approved to move this token by either approve or setApprovalForAll.

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

Emits a Transfer event.

Transfer(address from, address to, uint256 tokenId) event

Emitted when tokenId token is transferred from from to to.

Approval(address owner, address approved, uint256 tokenId) event

Emitted when owner enables approved to manage the tokenId token.

ApprovalForAll(address owner, address operator, bool approved) event

Emitted when owner enables or disables (approved) operator to manage all of its assets.

IERC721Metadata 

import "@openzeppelin/contracts/token/ERC721/extensions/IERC721Metadata.sol";

See https://eips.ethereum.org/EIPS/eip-721

Functions
IERC721
IERC165
Events
IERC721

name() → string external

Returns the token collection name.

symbol() → string external

Returns the token collection symbol.

tokenURI(uint256 tokenId) → string external

Returns the Uniform Resource Identifier (URI) for tokenId token.

IERC721Enumerable 

import "@openzeppelin/contracts/token/ERC721/extensions/IERC721Enumerable.sol";

See https://eips.ethereum.org/EIPS/eip-721

Functions
IERC721
IERC165
Events
IERC721

totalSupply() → uint256 external

Returns the total amount of tokens stored by the contract.

tokenOfOwnerByIndex(address owner, uint256 index) → uint256 tokenId external

Returns a token ID owned by owner at a given index of its token list. Use along with balanceOf to enumerate all of owner's tokens.

tokenByIndex(uint256 index) → uint256 external

Returns a token ID at a given index of all the tokens stored by the contract. Use along with totalSupply to enumerate all tokens.

ERC721 

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

Implementation of ERC721 Non-Fungible Token Standard, including the Metadata extension, but not including the Enumerable extension, which is available separately as ERC721Enumerable.

Functions
Events
IERC721

constructor(string name_, string symbol_) public

Initializes the contract by setting a name and a symbol to the token collection.

supportsInterface(bytes4 interfaceId) → bool public

See IERC165.supportsInterface.

balanceOf(address owner) → uint256 public

See IERC721.balanceOf.

ownerOf(uint256 tokenId) → address public

See IERC721.ownerOf.

name() → string public

See IERC721Metadata.name.

symbol() → string public

See IERC721Metadata.symbol.

tokenURI(uint256 tokenId) → string public

See IERC721Metadata.tokenURI.

_baseURI() → string internal

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 overriden in child contracts.

approve(address to, uint256 tokenId) public

See IERC721.approve.

getApproved(uint256 tokenId) → address public

See IERC721.getApproved.

setApprovalForAll(address operator, bool approved) public

See IERC721.setApprovalForAll.

isApprovedForAll(address owner, address operator) → bool public

See IERC721.isApprovedForAll.

transferFrom(address from, address to, uint256 tokenId) public

See IERC721.transferFrom.

safeTransferFrom(address from, address to, uint256 tokenId) public

See IERC721.safeTransferFrom.

safeTransferFrom(address from, address to, uint256 tokenId, bytes _data) public

See IERC721.safeTransferFrom.

_safeTransfer(address from, address to, uint256 tokenId, bytes _data) internal

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.

_exists(uint256 tokenId) → bool internal

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).

_isApprovedOrOwner(address spender, uint256 tokenId) → bool internal

Returns whether spender is allowed to manage tokenId.

Requirements:

  • tokenId must exist.

_safeMint(address to, uint256 tokenId) internal

Safely mints tokenId and transfers it to to.

Requirements:

Emits a Transfer event.

_safeMint(address to, uint256 tokenId, bytes _data) internal

Same as _safeMint, with an additional data parameter which is forwarded in IERC721Receiver.onERC721Received to contract recipients.

_mint(address to, uint256 tokenId) internal

Mints tokenId and transfers it to to.

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.

_burn(uint256 tokenId) internal

Destroys tokenId. The approval is cleared when the token is burned.

Requirements:

  • tokenId must exist.

Emits a Transfer event.

_transfer(address from, address to, uint256 tokenId) internal

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.

_approve(address to, uint256 tokenId) internal

Approve to to operate on tokenId

Emits a Approval event.

_beforeTokenTransfer(address from, address to, uint256 tokenId) internal

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 Using Hooks.

ERC721Enumerable 

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";

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.

Functions
ERC721
Events
IERC721

supportsInterface(bytes4 interfaceId) → bool public

See IERC165.supportsInterface.

tokenOfOwnerByIndex(address owner, uint256 index) → uint256 public

See IERC721Enumerable.tokenOfOwnerByIndex.

totalSupply() → uint256 public

See IERC721Enumerable.totalSupply.

tokenByIndex(uint256 index) → uint256 public

See IERC721Enumerable.tokenByIndex.

_beforeTokenTransfer(address from, address to, uint256 tokenId) internal

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 Using Hooks.

IERC721Receiver 

import "@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";

Interface for any contract that wants to support safeTransfers from ERC721 asset contracts.

Functions

onERC721Received(address operator, address from, uint256 tokenId, bytes data) → bytes4 external

Whenever an IERC721 tokenId token is transferred to this contract via IERC721.safeTransferFrom by operator from from, this function is called.

It must return its Solidity selector to confirm the token transfer. If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.

The selector can be obtained in Solidity with IERC721.onERC721Received.selector.

Extensions

ERC721Pausable 

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Pausable.sol";

ERC721 token with pausable token transfers, minting and burning.

Useful for scenarios such as preventing trades until the end of an evaluation period, or having an emergency switch for freezing all token transfers in the event of a large bug.

Functions
Pausable
ERC721
Events
Pausable
IERC721

_beforeTokenTransfer(address from, address to, uint256 tokenId) internal

See ERC721._beforeTokenTransfer.

Requirements:

  • the contract must not be paused.

ERC721Burnable 

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Burnable.sol";

ERC721 Token that can be irreversibly burned (destroyed).

Functions
ERC721
Events
IERC721

burn(uint256 tokenId) public

Burns tokenId. See ERC721._burn.

Requirements:

  • The caller must own tokenId or be an approved operator.

ERC721URIStorage 

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";

ERC721 token with storage based token URI management.

Functions
ERC721
Events
IERC721

tokenURI(uint256 tokenId) → string public

See IERC721Metadata.tokenURI.

_setTokenURI(uint256 tokenId, string _tokenURI) internal

Sets _tokenURI as the tokenURI of tokenId.

Requirements:

  • tokenId must exist.

_burn(uint256 tokenId) internal

Destroys tokenId. The approval is cleared when the token is burned.

Requirements:

  • tokenId must exist.

Emits a Transfer event.

Presets

These contracts are preconfigured combinations of the above features. They can be used through inheritance or as models to copy and paste their source code.

ERC721PresetMinterPauserAutoId 

import "@openzeppelin/contracts/token/ERC721/presets/ERC721PresetMinterPauserAutoId.sol";

ERC721 token, including:

  • ability for holders to burn (destroy) their tokens

  • a minter role that allows for token minting (creation)

  • a pauser role that allows to stop all token transfers

  • token ID and URI autogeneration

This contract uses AccessControl to lock permissioned functions using the different roles - head to its documentation for details.

The account that deploys the contract will be granted the minter and pauser roles, as well as the default admin role, which will let it grant both minter and pauser roles to other accounts.

Functions
Pausable
ERC721Burnable
ERC721Enumerable
ERC721
AccessControlEnumerable
AccessControl
Events
Pausable
IERC721
IAccessControl

constructor(string name, string symbol, string baseTokenURI) public

Grants DEFAULT_ADMIN_ROLE, MINTER_ROLE and PAUSER_ROLE to the account that deploys the contract.

Token URIs will be autogenerated based on baseURI and their token IDs. See ERC721.tokenURI.

_baseURI() → string internal

mint(address to) public

Creates a new token for to. Its token ID will be automatically assigned (and available on the emitted IERC721.Transfer event), and the token URI autogenerated based on the base URI passed at construction.

See ERC721._mint.

Requirements:

  • the caller must have the MINTER_ROLE.

pause() public

Pauses all token transfers.

See ERC721Pausable and Pausable._pause.

Requirements:

  • the caller must have the PAUSER_ROLE.

unpause() public

Unpauses all token transfers.

See ERC721Pausable and Pausable._unpause.

Requirements:

  • the caller must have the PAUSER_ROLE.

_beforeTokenTransfer(address from, address to, uint256 tokenId) internal

supportsInterface(bytes4 interfaceId) → bool public

See IERC165.supportsInterface.

Utilities

ERC721Holder 

import "@openzeppelin/contracts/token/ERC721/utils/ERC721Holder.sol";

Implementation of the IERC721Receiver interface.

Accepts all token transfers. Make sure the contract is able to use its token with IERC721.safeTransferFrom, IERC721.approve or IERC721.setApprovalForAll.

Functions

onERC721Received(address, address, uint256, bytes) → bytes4 public

See IERC721Receiver.onERC721Received.

Always returns IERC721Receiver.onERC721Received.selector.

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