ERC 20

This set of interfaces, contracts, and utilities are all related to the ERC20 Token Standard.

For an overview of ERC20 tokens and a walk through on how to create a token contract read our ERC20 guide.

There a few core contracts that implement the behavior specified in the EIP:

  • IERC20: the interface all ERC20 implementations should conform to.

  • IERC20Metadata: the extended ERC20 interface including the name, symbol and decimals functions.

  • ERC20: the implementation of the ERC20 interface, including the name, symbol and decimals optional standard extension to the base interface.

Additionally there are multiple custom extensions, including:

  • ERC20Burnable: destruction of own tokens.

  • ERC20Capped: enforcement of a cap to the total supply when minting tokens.

  • ERC20Pausable: ability to pause token transfers.

  • ERC20Snapshot: efficient storage of past token balances to be later queried at any point in time.

  • ERC20Permit: gasless approval of tokens (standardized as ERC2612).

  • ERC20FlashMint: token level support for flash loans through the minting and burning of ephemeral tokens (standardized as ERC3156).

  • ERC20Votes: support for voting and vote delegation.

  • ERC20VotesComp: support for voting and vote delegation (compatible with Compound’s token, with uint96 restrictions).

  • ERC20Wrapper: wrapper to create an ERC20 backed by another ERC20, with deposit and withdraw methods. Useful in conjunction with ERC20Votes.

Finally, there are some utilities to interact with ERC20 contracts in various ways.

  • SafeERC20: a wrapper around the interface that eliminates the need to handle boolean return values.

  • TokenTimelock: hold tokens for a beneficiary until a specified time.

The following related EIPs are in draft status.

This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC20 (such as _mint) and expose them as external functions in the way they prefer. On the other hand, ERC20 Presets (such as ERC20PresetMinterPauser) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.

Core

IERC20

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

Interface of the ERC20 standard as defined in the EIP.

totalSupply() → uint256 external

Returns the amount of tokens in existence.

balanceOf(address account) → uint256 external

Returns the amount of tokens owned by account.

transfer(address recipient, uint256 amount) → bool external

Moves amount tokens from the caller’s account to recipient.

Returns a boolean value indicating whether the operation succeeded.

Emits a Transfer event.

allowance(address owner, address spender) → uint256 external

Returns the remaining number of tokens that spender will be allowed to spend on behalf of owner through transferFrom. This is zero by default.

This value changes when approve or transferFrom are called.

approve(address spender, uint256 amount) → bool external

Sets amount as the allowance of spender over the caller’s tokens.

Returns a boolean value indicating whether the operation succeeded.

Beware that changing an allowance with this method brings the risk that someone may use both the old and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this race condition is to first reduce the spender’s allowance to 0 and set the desired value afterwards: https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729

Emits an Approval event.

transferFrom(address sender, address recipient, uint256 amount) → bool external

Moves amount tokens from sender to recipient using the allowance mechanism. amount is then deducted from the caller’s allowance.

Returns a boolean value indicating whether the operation succeeded.

Emits a Transfer event.

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

Emitted when value tokens are moved from one account (from) to another (to).

Note that value may be zero.

Approval(address owner, address spender, uint256 value) event

Emitted when the allowance of a spender for an owner is set by a call to approve. value is the new allowance.

IERC20Metadata

import "@openzeppelin/contracts/token/ERC20/extensions/IERC20Metadata.sol";

Interface for the optional metadata functions from the ERC20 standard.

Available since v4.1.

name() → string external

Returns the name of the token.

symbol() → string external

Returns the symbol of the token.

decimals() → uint8 external

Returns the decimals places of the token.

ERC20

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

Implementation of the IERC20 interface.

This implementation is agnostic to the way tokens are created. This means that a supply mechanism has to be added in a derived contract using _mint. For a generic mechanism see ERC20PresetMinterPauser.

For a detailed writeup see our guide How to implement supply mechanisms.

We have followed general OpenZeppelin Contracts guidelines: functions revert instead returning false on failure. This behavior is nonetheless conventional and does not conflict with the expectations of ERC20 applications.

Additionally, an Approval event is emitted on calls to transferFrom. This allows applications to reconstruct the allowance for all accounts just by listening to said events. Other implementations of the EIP may not emit these events, as it isn’t required by the specification.

Finally, the non-standard decreaseAllowance and increaseAllowance functions have been added to mitigate the well-known issues around setting allowances. See IERC20.approve.

constructor(string name_, string symbol_) public

Sets the values for name and symbol.

The default value of decimals is 18. To select a different value for decimals you should overload it.

All two of these values are immutable: they can only be set once during construction.

name() → string public

Returns the name of the token.

symbol() → string public

Returns the symbol of the token, usually a shorter version of the name.

decimals() → uint8 public

Returns the number of decimals used to get its user representation. For example, if decimals equals 2, a balance of 505 tokens should be displayed to a user as 5.05 (505 / 10 ** 2).

Tokens usually opt for a value of 18, imitating the relationship between Ether and Wei. This is the value ERC20 uses, unless this function is overridden;

This information is only used for display purposes: it in no way affects any of the arithmetic of the contract, including IERC20.balanceOf and IERC20.Transfer.

totalSupply() → uint256 public

balanceOf(address account) → uint256 public

transfer(address recipient, uint256 amount) → bool public

Requirements:

  • recipient cannot be the zero address.

  • the caller must have a balance of at least amount.

allowance(address owner, address spender) → uint256 public

approve(address spender, uint256 amount) → bool public

Requirements:

  • spender cannot be the zero address.

transferFrom(address sender, address recipient, uint256 amount) → bool public

Emits an Approval event indicating the updated allowance. This is not required by the EIP. See the note at the beginning of ERC20.

Requirements:

  • sender and recipient cannot be the zero address.

  • sender must have a balance of at least amount.

  • the caller must have allowance for sender's tokens of at least amount.

increaseAllowance(address spender, uint256 addedValue) → bool public

Atomically increases the allowance granted to spender by the caller.

This is an alternative to approve that can be used as a mitigation for problems described in IERC20.approve.

Emits an Approval event indicating the updated allowance.

Requirements:

  • spender cannot be the zero address.

decreaseAllowance(address spender, uint256 subtractedValue) → bool public

Atomically decreases the allowance granted to spender by the caller.

This is an alternative to approve that can be used as a mitigation for problems described in IERC20.approve.

Emits an Approval event indicating the updated allowance.

Requirements:

  • spender cannot be the zero address.

  • spender must have allowance for the caller of at least subtractedValue.

_transfer(address sender, address recipient, uint256 amount) internal

Moves amount of tokens from sender to recipient.

This internal function is equivalent to transfer, and can be used to e.g. implement automatic token fees, slashing mechanisms, etc.

Emits a transfer event.

Requirements:

  • sender cannot be the zero address.

  • recipient cannot be the zero address.

  • sender must have a balance of at least amount.

_mint(address account, uint256 amount) internal

Creates amount tokens and assigns them to account, increasing the total supply.

Emits a transfer event with from set to the zero address.

Requirements:

  • account cannot be the zero address.

_burn(address account, uint256 amount) internal

Destroys amount tokens from account, reducing the total supply.

Emits a transfer event with to set to the zero address.

Requirements:

  • account cannot be the zero address.

  • account must have at least amount tokens.

_approve(address owner, address spender, uint256 amount) internal

Sets amount as the allowance of spender over the owner s tokens.

This internal function is equivalent to approve, and can be used to e.g. set automatic allowances for certain subsystems, etc.

Emits an Approval event.

Requirements:

  • owner cannot be the zero address.

  • spender cannot be the zero address.

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

Hook that is called before any transfer of tokens. This includes minting and burning.

Calling conditions:

  • when from and to are both non-zero, amount of from's tokens will be transferred to to.

  • when from is zero, amount tokens will be minted for to.

  • when to is zero, amount of from's tokens will be burned.

  • from and to are never both zero.

To learn more about hooks, head to Using Hooks.

_afterTokenTransfer(address from, address to, uint256 amount) internal

Hook that is called after any transfer of tokens. This includes minting and burning.

Calling conditions:

  • when from and to are both non-zero, amount of from's tokens has been transferred to to.

  • when from is zero, amount tokens have been minted for to.

  • when to is zero, amount of from's tokens have been burned.

  • from and to are never both zero.

To learn more about hooks, head to Using Hooks.

Extensions

ERC20Burnable

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";

Extension of ERC20 that allows token holders to destroy both their own tokens and those that they have an allowance for, in a way that can be recognized off-chain (via event analysis).

burn(uint256 amount) public

Destroys amount tokens from the caller.

burnFrom(address account, uint256 amount) public

Destroys amount tokens from account, deducting from the caller’s allowance.

Requirements:

  • the caller must have allowance for accounts's tokens of at least amount.

ERC20Capped

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Capped.sol";

Extension of ERC20 that adds a cap to the supply of tokens.

constructor(uint256 cap_) internal

Sets the value of the cap. This value is immutable, it can only be set once during construction.

cap() → uint256 public

Returns the cap on the token’s total supply.

_mint(address account, uint256 amount) internal

ERC20Pausable

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Pausable.sol";

ERC20 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.

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

Requirements:

  • the contract must not be paused.

ERC20Snapshot

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Snapshot.sol";

This contract extends an ERC20 token with a snapshot mechanism. When a snapshot is created, the balances and total supply at the time are recorded for later access.

This can be used to safely create mechanisms based on token balances such as trustless dividends or weighted voting. In naive implementations it’s possible to perform a "double spend" attack by reusing the same balance from different accounts. By using snapshots to calculate dividends or voting power, those attacks no longer apply. It can also be used to create an efficient ERC20 forking mechanism.

Snapshots are created by the internal _snapshot function, which will emit the Snapshot event and return a snapshot id. To get the total supply at the time of a snapshot, call the function totalSupplyAt with the snapshot id. To get the balance of an account at the time of a snapshot, call the balanceOfAt function with the snapshot id and the account address.

Snapshot policy can be customized by overriding the _getCurrentSnapshotId method. For example, having it return block.number will trigger the creation of snapshot at the begining of each new block. When overridding this function, be careful about the monotonicity of its result. Non-monotonic snapshot ids will break the contract.

Implementing snapshots for every block using this method will incur significant gas costs. For a gas-efficient alternative consider ERC20Votes.

Gas Costs

Snapshots are efficient. Snapshot creation is O(1). Retrieval of balances or total supply from a snapshot is O(log n) in the number of snapshots that have been created, although n for a specific account will generally be much smaller since identical balances in subsequent snapshots are stored as a single entry.

There is a constant overhead for normal ERC20 transfers due to the additional snapshot bookkeeping. This overhead is only significant for the first transfer that immediately follows a snapshot for a particular account. Subsequent transfers will have normal cost until the next snapshot, and so on.

_snapshot() → uint256 internal

Creates a new snapshot and returns its snapshot id.

Emits a Snapshot event that contains the same id.

_snapshot is internal and you have to decide how to expose it externally. Its usage may be restricted to a set of accounts, for example using AccessControl, or it may be open to the public.

While an open way of calling _snapshot is required for certain trust minimization mechanisms such as forking, you must consider that it can potentially be used by attackers in two ways.

First, it can be used to increase the cost of retrieval of values from snapshots, although it will grow logarithmically thus rendering this attack ineffective in the long term. Second, it can be used to target specific accounts and increase the cost of ERC20 transfers for them, in the ways specified in the Gas Costs section above.

We haven’t measured the actual numbers; if this is something you’re interested in please reach out to us.

_getCurrentSnapshotId() → uint256 internal

Get the current snapshotId

balanceOfAt(address account, uint256 snapshotId) → uint256 public

Retrieves the balance of account at the time snapshotId was created.

totalSupplyAt(uint256 snapshotId) → uint256 public

Retrieves the total supply at the time snapshotId was created.

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

Snapshot(uint256 id) event

Emitted by _snapshot when a snapshot identified by id is created.

ERC20Votes

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Votes.sol";

Extension of ERC20 to support Compound-like voting and delegation. This version is more generic than Compound’s, and supports token supply up to 2224 - 1, while COMP is limited to 296 - 1.

If exact COMP compatibility is required, use the ERC20VotesComp variant of this module.

This extension keeps a history (checkpoints) of each account’s vote power. Vote power can be delegated either by calling the delegate function directly, or by providing a signature to be used with delegateBySig. Voting power can be queried through the public accessors getVotes and getPastVotes.

By default, token balance does not account for voting power. This makes transfers cheaper. The downside is that it requires users to delegate to themselves in order to activate checkpoints and have their voting power tracked. Enabling self-delegation can easily be done by overriding the delegates function. Keep in mind however that this will significantly increase the base gas cost of transfers.

Available since v4.2.

checkpoints(address account, uint32 pos) → struct ERC20Votes.Checkpoint public

Get the pos-th checkpoint for account.

numCheckpoints(address account) → uint32 public

Get number of checkpoints for account.

delegates(address account) → address public

Get the address account is currently delegating to.

getVotes(address account) → uint256 public

Gets the current votes balance for account

getPastVotes(address account, uint256 blockNumber) → uint256 public

Retrieve the number of votes for account at the end of blockNumber.

Requirements:

  • blockNumber must have been already mined

getPastTotalSupply(uint256 blockNumber) → uint256 public

Retrieve the totalSupply at the end of blockNumber. Note, this value is the sum of all balances. It is but NOT the sum of all the delegated votes!

Requirements:

  • blockNumber must have been already mined

delegate(address delegatee) public

Delegate votes from the sender to delegatee.

delegateBySig(address delegatee, uint256 nonce, uint256 expiry, uint8 v, bytes32 r, bytes32 s) public

Delegates votes from signer to delegatee

_maxSupply() → uint224 internal

Maximum token supply. Defaults to type(uint224).max (2224 - 1).

_mint(address account, uint256 amount) internal

Snapshots the totalSupply after it has been increased.

_burn(address account, uint256 amount) internal

Snapshots the totalSupply after it has been decreased.

_afterTokenTransfer(address from, address to, uint256 amount) internal

Move voting power when tokens are transferred.

Emits a DelegateVotesChanged event.

_delegate(address delegator, address delegatee) internal

Change delegation for delegator to delegatee.

DelegateChanged(address delegator, address fromDelegate, address toDelegate) event

Emitted when an account changes their delegate.

DelegateVotesChanged(address delegate, uint256 previousBalance, uint256 newBalance) event

Emitted when a token transfer or delegate change results in changes to an account’s voting power.

ERC20VotesComp

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20VotesComp.sol";

Extension of ERC20 to support Compound’s voting and delegation. This version exactly matches Compound’s interface, with the drawback of only supporting supply up to (296 - 1).

You should use this contract if you need exact compatibility with COMP (for example in order to use your token with Governor Alpha or Bravo) and if you are sure the supply cap of 296 is enough for you. Otherwise, use the ERC20Votes variant of this module.

This extension keeps a history (checkpoints) of each account’s vote power. Vote power can be delegated either by calling the delegate function directly, or by providing a signature to be used with delegateBySig. Voting power can be queried through the public accessors getCurrentVotes and getPriorVotes.

By default, token balance does not account for voting power. This makes transfers cheaper. The downside is that it requires users to delegate to themselves in order to activate checkpoints and have their voting power tracked. Enabling self-delegation can easily be done by overriding the delegates function. Keep in mind however that this will significantly increase the base gas cost of transfers.

Available since v4.2.

getCurrentVotes(address account) → uint96 external

Comp version of the getVotes accessor, with uint96 return type.

getPriorVotes(address account, uint256 blockNumber) → uint96 external

Comp version of the getPastVotes accessor, with uint96 return type.

_maxSupply() → uint224 internal

Maximum token supply. Reduced to type(uint96).max (296 - 1) to fit COMP interface.

ERC20Wrapper

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Wrapper.sol";

Extension of the ERC20 token contract to support token wrapping.

Users can deposit and withdraw "underlying tokens" and receive a matching number of "wrapped tokens". This is useful in conjunction with other modules. For example, combining this wrapping mechanism with ERC20Votes will allow the wrapping of an existing "basic" ERC20 into a governance token.

Available since v4.2.

constructor(contract IERC20 underlyingToken) internal

depositFor(address account, uint256 amount) → bool public

Allow a user to deposit underlying tokens and mint the corresponding number of wrapped tokens.

withdrawTo(address account, uint256 amount) → bool public

Allow a user to burn a number of wrapped tokens and withdraw the corresponding number of underlying tokens.

_recover(address account) → uint256 internal

Mint wrapped token to cover any underlyingTokens that would have been transfered by mistake. Internal function that can be exposed with access control if desired.

ERC20FlashMint

import "@openzeppelin/contracts/token/ERC20/extensions/ERC20FlashMint.sol";

Implementation of the ERC3156 Flash loans extension, as defined in ERC-3156.

Adds the flashLoan method, which provides flash loan support at the token level. By default there is no fee, but this can be changed by overriding flashFee.

Available since v4.1.

maxFlashLoan(address token) → uint256 public

Returns the maximum amount of tokens available for loan.

flashFee(address token, uint256 amount) → uint256 public

Returns the fee applied when doing flash loans. By default this implementation has 0 fees. This function can be overloaded to make the flash loan mechanism deflationary.

flashLoan(contract IERC3156FlashBorrower receiver, address token, uint256 amount, bytes data) → bool public

Performs a flash loan. New tokens are minted and sent to the receiver, who is required to implement the IERC3156FlashBorrower interface. By the end of the flash loan, the receiver is expected to own amount + fee tokens and have them approved back to the token contract itself so they can be burned.

Draft EIPs

The following EIPs are still in Draft status. Due to their nature as drafts, the details of these contracts may change and we cannot guarantee their stability. Minor releases of OpenZeppelin Contracts may contain breaking changes for the contracts in this directory, which will be duly announced in the changelog. The EIPs included here are used by projects in production and this may make them less likely to change significantly.

ERC20Permit

import "@openzeppelin/contracts/token/ERC20/extensions/draft-ERC20Permit.sol";

Implementation of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in EIP-2612.

Adds the permit method, which can be used to change an account’s ERC20 allowance (see IERC20.allowance) by presenting a message signed by the account. By not relying on IERC20.approve, the token holder account doesn’t need to send a transaction, and thus is not required to hold Ether at all.

Available since v3.4.

constructor(string name) internal

Initializes the EIP712 domain separator using the name parameter, and setting version to "1".

It’s a good idea to use the same name that is defined as the ERC20 token name.

permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) public

nonces(address owner) → uint256 public

DOMAIN_SEPARATOR() → bytes32 external

_useNonce(address owner) → uint256 current internal

"Consume a nonce": return the current value and increment.

Available since v4.1.

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.

ERC20PresetMinterPauser

import "@openzeppelin/contracts/token/ERC20/presets/ERC20PresetMinterPauser.sol";

ERC20 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

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.

constructor(string name, string symbol) public

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

mint(address to, uint256 amount) public

Creates amount new tokens for to.

Requirements:

  • the caller must have the MINTER_ROLE.

pause() public

Pauses all token transfers.

Requirements:

  • the caller must have the PAUSER_ROLE.

unpause() public

Unpauses all token transfers.

Requirements:

  • the caller must have the PAUSER_ROLE.

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

ERC20PresetFixedSupply

import "@openzeppelin/contracts/token/ERC20/presets/ERC20PresetFixedSupply.sol";

ERC20 token, including:

  • Preminted initial supply

  • Ability for holders to burn (destroy) their tokens

  • No access control mechanism (for minting/pausing) and hence no governance

This contract uses ERC20Burnable to include burn capabilities - head to its documentation for details.

Available since v3.4.

constructor(string name, string symbol, uint256 initialSupply, address owner) public

Mints initialSupply amount of token and transfers them to owner.

Utilities

SafeERC20

import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

Wrappers around ERC20 operations that throw on failure (when the token contract returns false). Tokens that return no value (and instead revert or throw on failure) are also supported, non-reverting calls are assumed to be successful. To use this library you can add a using SafeERC20 for IERC20; statement to your contract, which allows you to call the safe operations as token.safeTransfer(…​), etc.

safeTransfer(contract IERC20 token, address to, uint256 value) internal

safeTransferFrom(contract IERC20 token, address from, address to, uint256 value) internal

safeApprove(contract IERC20 token, address spender, uint256 value) internal

Deprecated. This function has issues similar to the ones found in IERC20.approve, and its usage is discouraged.

Whenever possible, use safeIncreaseAllowance and safeDecreaseAllowance instead.

safeIncreaseAllowance(contract IERC20 token, address spender, uint256 value) internal

safeDecreaseAllowance(contract IERC20 token, address spender, uint256 value) internal

TokenTimelock

import "@openzeppelin/contracts/token/ERC20/utils/TokenTimelock.sol";

A token holder contract that will allow a beneficiary to extract the tokens after a given release time.

Useful for simple vesting schedules like "advisors get all of their tokens after 1 year".

constructor(contract IERC20 token_, address beneficiary_, uint256 releaseTime_) public

token() → contract IERC20 public

beneficiary() → address public

releaseTime() → uint256 public

release() public