ERC 20

This document is better viewed at https://docs.openzeppelin.com/contracts/api/token/erc20

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 are 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:

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

  • 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.

  • 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.

  • ERC4626: tokenized vault that manages shares (represented as ERC20) that are backed by assets (another ERC20).

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.

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.

Functions
Events

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 to, uint256 amount) → bool external

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

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 from, address to, uint256 amount) → bool external

Moves amount tokens from from to to 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 indexed from, address indexed 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 indexed owner, address indexed 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.

Functions
IERC20
Events
IERC20

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.

The default value of decimals is 18. To change this, you should override this function so it returns a different value.

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.

Functions
Events
IERC20

constructor(string name_, string symbol_) public

Sets the values for name and symbol.

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 default value returned by this function, unless it’s 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

See IERC20.totalSupply.

balanceOf(address account) → uint256 public

See IERC20.balanceOf.

transfer(address to, uint256 amount) → bool public

See IERC20.transfer.

Requirements:

  • to cannot be the zero address.

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

allowance(address owner, address spender) → uint256 public

See IERC20.allowance.

approve(address spender, uint256 amount) → bool public

See IERC20.approve.

If amount is the maximum uint256, the allowance is not updated on transferFrom. This is semantically equivalent to an infinite approval.

Requirements:

  • spender cannot be the zero address.

transferFrom(address from, address to, uint256 amount) → bool public

See IERC20.transferFrom.

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

Does not update the allowance if the current allowance is the maximum uint256.

Requirements:

  • from and to cannot be the zero address.

  • from must have a balance of at least amount.

  • the caller must have allowance for from'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 from, address to, uint256 amount) internal

Moves amount of tokens from from to to.

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:

  • from cannot be the zero address.

  • to cannot be the zero address.

  • from 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.

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

Updates owner s allowance for spender based on spent amount.

Does not update the allowance amount in case of infinite allowance. Revert if not enough allowance is available.

Might emit an Approval event.

_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

IERC20Permit 

import "@openzeppelin/contracts/token/ERC20/extensions/IERC20Permit.sol";

Interface 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.

Security Considerations

There are two important considerations concerning the use of permit. The first is that a valid permit signature expresses an allowance, and it should not be assumed to convey additional meaning. In particular, it should not be considered as an intention to spend the allowance in any specific way. The second is that because permits have built-in replay protection and can be submitted by anyone, they can be frontrun. A protocol that uses permits should take this into consideration and allow a permit call to fail. Combining these two aspects, a pattern that may be generally recommended is:

function doThingWithPermit(..., uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) public {
    try token.permit(msg.sender, address(this), value, deadline, v, r, s) {} catch {}
    doThing(..., value);
}

function doThing(..., uint256 value) public {
    token.safeTransferFrom(msg.sender, address(this), value);
    ...
}

Observe that: 1) msg.sender is used as the owner, leaving no ambiguity as to the signer intent, and 2) the use of try/catch allows the permit to fail and makes the code tolerant to frontrunning. (See also SafeERC20.safeTransferFrom).

Additionally, note that smart contract wallets (such as Argent or Safe) are not able to produce permit signatures, so contracts should have entry points that don’t rely on permit.

Functions

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

Sets value as the allowance of spender over owner's tokens, given owner's signed approval.

The same issues IERC20.approve has related to transaction ordering also apply here.

Emits an Approval event.

Requirements:

  • spender cannot be the zero address.

  • deadline must be a timestamp in the future.

  • v, r and s must be a valid secp256k1 signature from owner over the EIP712-formatted function arguments.

  • the signature must use owner's current nonce (see nonces).

For more information on the signature format, see the relevant EIP section.

See Security Considerations above.

nonces(address owner) → uint256 external

Returns the current nonce for owner. This value must be included whenever a signature is generated for permit.

Every successful call to permit increases owner's nonce by one. This prevents a signature from being used multiple times.

DOMAIN_SEPARATOR() → bytes32 external

Returns the domain separator used in the encoding of the signature for permit, as defined by EIP712.

ERC20Permit 

import "@openzeppelin/contracts/token/ERC20/extensions/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.

Functions
EIP712
ERC20
Events
IERC5267
IERC20

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

Sets value as the allowance of spender over owner's tokens, given owner's signed approval.

The same issues IERC20.approve has related to transaction ordering also apply here.

Emits an Approval event.

Requirements:

  • spender cannot be the zero address.

  • deadline must be a timestamp in the future.

  • v, r and s must be a valid secp256k1 signature from owner over the EIP712-formatted function arguments.

  • the signature must use owner's current nonce (see nonces).

For more information on the signature format, see the relevant EIP section.

See Security Considerations above.

nonces(address owner) → uint256 public

Returns the current nonce for owner. This value must be included whenever a signature is generated for permit.

Every successful call to permit increases owner's nonce by one. This prevents a signature from being used multiple times.

DOMAIN_SEPARATOR() → bytes32 external

Returns the domain separator used in the encoding of the signature for permit, as defined by EIP712.

_useNonce(address owner) → uint256 current internal

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

Available since v4.1.

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).

Functions
ERC20
Events
IERC20

burn(uint256 amount) public

Destroys amount tokens from the caller.

See ERC20._burn.

burnFrom(address account, uint256 amount) public

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

See ERC20._burn and ERC20.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.

Functions
ERC20
Events
IERC20

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

See ERC20._mint.

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.

This contract does not include public pause and unpause functions. In addition to inheriting this contract, you must define both functions, invoking the Pausable._pause and Pausable._unpause internal functions, with appropriate access control, e.g. using AccessControl or Ownable. Not doing so will make the contract unpausable.
Functions
Pausable
ERC20
Events
Pausable
IERC20

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

See ERC20._beforeTokenTransfer.

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 beginning of each new block. When overriding 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.

Functions
ERC20
Events
IERC20

_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

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.

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.

Available since v4.2.

Functions
ERC20Permit
EIP712
ERC20
Events
IVotes
IERC5267
IERC20

clock() → uint48 public

Clock used for flagging checkpoints. Can be overridden to implement timestamp based checkpoints (and voting).

CLOCK_MODE() → string public

Description of the clock

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 timepoint) → uint256 public

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

Requirements:

  • timepoint must be in the past

getPastTotalSupply(uint256 timepoint) → uint256 public

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

Requirements:

  • timepoint must be in the past

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 IVotes.DelegateVotesChanged event.

_delegate(address delegator, address delegatee) internal

Change delegation for delegator to delegatee.

Emits events IVotes.DelegateChanged and IVotes.DelegateVotesChanged.

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.

Available since v4.2.

Functions
ERC20Votes
ERC20Permit
EIP712
ERC20
Events
IVotes
IERC5267
IERC20

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.

Functions
ERC20
Events
IERC20

constructor(contract IERC20 underlyingToken) internal

decimals() → uint8 public

See ERC20.decimals.

underlying() → contract IERC20 public

Returns the address of the underlying ERC-20 token that is being wrapped.

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 transferred 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.

Functions
ERC20
Events
IERC20

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. This function calls the _flashFee function which returns the fee applied when doing flash loans.

_flashFee(address token, uint256 amount) → uint256 internal

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.

_flashFeeReceiver() → address internal

Returns the receiver address of the flash fee. By default this implementation returns the address(0) which means the fee amount will be burnt. This function can be overloaded to change the fee receiver.

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.

ERC4626 

import "@openzeppelin/contracts/token/ERC20/extensions/ERC4626.sol";

Implementation of the ERC4626 "Tokenized Vault Standard" as defined in EIP-4626.

This extension allows the minting and burning of "shares" (represented using the ERC20 inheritance) in exchange for underlying "assets" through standardized deposit, mint, redeem and burn workflows. This contract extends the ERC20 standard. Any additional extensions included along it would affect the "shares" token represented by this contract and not the "assets" token which is an independent contract.

In empty (or nearly empty) ERC-4626 vaults, deposits are at high risk of being stolen through frontrunning with a "donation" to the vault that inflates the price of a share. This is variously known as a donation or inflation attack and is essentially a problem of slippage. Vault deployers can protect against this attack by making an initial deposit of a non-trivial amount of the asset, such that price manipulation becomes infeasible. Withdrawals may similarly be affected by slippage. Users can protect against this attack as well as unexpected slippage in general by verifying the amount received is as expected, using a wrapper that performs these checks such as ERC4626Router.

Since v4.9, this implementation uses virtual assets and shares to mitigate that risk. The _decimalsOffset() corresponds to an offset in the decimal representation between the underlying asset’s decimals and the vault decimals. This offset also determines the rate of virtual shares to virtual assets in the vault, which itself determines the initial exchange rate. While not fully preventing the attack, analysis shows that the default offset (0) makes it non-profitable, as a result of the value being captured by the virtual shares (out of the attacker’s donation) matching the attacker’s expected gains. With a larger offset, the attack becomes orders of magnitude more expensive than it is profitable. More details about the underlying math can be found here.

The drawback of this approach is that the virtual shares do capture (a very small) part of the value being accrued to the vault. Also, if the vault experiences losses, the users try to exit the vault, the virtual shares and assets will cause the first user to exit to experience reduced losses in detriment to the last users that will experience bigger losses. Developers willing to revert back to the pre-v4.9 behavior just need to override the _convertToShares and _convertToAssets functions.

To learn more, check out our ERC-4626 guide.

Available since v4.7.

Functions
ERC20
Events
IERC4626
IERC20

constructor(contract IERC20 asset_) internal

Set the underlying asset contract. This must be an ERC20-compatible contract (ERC20 or ERC777).

decimals() → uint8 public

Decimals are computed by adding the decimal offset on top of the underlying asset’s decimals. This "original" value is cached during construction of the vault contract. If this read operation fails (e.g., the asset has not been created yet), a default of 18 is used to represent the underlying asset’s decimals.

See IERC20Metadata.decimals.

asset() → address public

See IERC4626.asset.

totalAssets() → uint256 public

See IERC4626.totalAssets.

convertToShares(uint256 assets) → uint256 public

See IERC4626.convertToShares.

convertToAssets(uint256 shares) → uint256 public

See IERC4626.convertToAssets.

maxDeposit(address) → uint256 public

See IERC4626.maxDeposit.

maxMint(address) → uint256 public

See IERC4626.maxMint.

maxWithdraw(address owner) → uint256 public

See IERC4626.maxWithdraw.

maxRedeem(address owner) → uint256 public

See IERC4626.maxRedeem.

previewDeposit(uint256 assets) → uint256 public

See IERC4626.previewDeposit.

previewMint(uint256 shares) → uint256 public

See IERC4626.previewMint.

previewWithdraw(uint256 assets) → uint256 public

See IERC4626.previewWithdraw.

previewRedeem(uint256 shares) → uint256 public

See IERC4626.previewRedeem.

deposit(uint256 assets, address receiver) → uint256 public

See IERC4626.deposit.

mint(uint256 shares, address receiver) → uint256 public

See IERC4626.mint.

As opposed to deposit, minting is allowed even if the vault is in a state where the price of a share is zero. In this case, the shares will be minted without requiring any assets to be deposited.

withdraw(uint256 assets, address receiver, address owner) → uint256 public

See IERC4626.withdraw.

redeem(uint256 shares, address receiver, address owner) → uint256 public

See IERC4626.redeem.

_convertToShares(uint256 assets, enum Math.Rounding rounding) → uint256 internal

Internal conversion function (from assets to shares) with support for rounding direction.

_convertToAssets(uint256 shares, enum Math.Rounding rounding) → uint256 internal

Internal conversion function (from shares to assets) with support for rounding direction.

_deposit(address caller, address receiver, uint256 assets, uint256 shares) internal

Deposit/mint common workflow.

_withdraw(address caller, address receiver, address owner, uint256 assets, uint256 shares) internal

Withdraw/redeem common workflow.

_decimalsOffset() → uint8 internal

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.

Deprecated in favor of Contracts Wizard.

Functions
Pausable
ERC20Burnable
ERC20
AccessControlEnumerable
AccessControl
Events
Pausable
IERC20
IAccessControl

constructor(string name, string symbol) public

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

See ERC20.constructor.

mint(address to, uint256 amount) public

Creates amount new tokens for to.

See ERC20._mint.

Requirements:

  • the caller must have the MINTER_ROLE.

pause() public

Pauses all token transfers.

See ERC20Pausable and Pausable._pause.

Requirements:

  • the caller must have the PAUSER_ROLE.

unpause() public

Unpauses all token transfers.

See ERC20Pausable and Pausable._unpause.

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.

Deprecated in favor of Contracts Wizard.

Functions
ERC20Burnable
ERC20
Events
IERC20

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

Mints initialSupply amount of token and transfers them to owner.

See ERC20.constructor.

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.

Functions

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

Transfer value amount of token from the calling contract to to. If token returns no value, non-reverting calls are assumed to be successful.

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

Transfer value amount of token from from to to, spending the approval given by from to the calling contract. If token returns no value, non-reverting calls are assumed to be successful.

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

Increase the calling contract’s allowance toward spender by value. If token returns no value, non-reverting calls are assumed to be successful.

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

Decrease the calling contract’s allowance toward spender by value. If token returns no value, non-reverting calls are assumed to be successful.

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

Set the calling contract’s allowance toward spender to value. If token returns no value, non-reverting calls are assumed to be successful. Meant to be used with tokens that require the approval to be set to zero before setting it to a non-zero value, such as USDT.

safePermit(contract IERC20Permit token, address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) internal

Use a ERC-2612 signature to set the owner approval toward spender on token. Revert on invalid signature.

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".

Functions

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

Deploys a timelock instance that is able to hold the token specified, and will only release it to beneficiary_ when release is invoked after releaseTime_. The release time is specified as a Unix timestamp (in seconds).

token() → contract IERC20 public

Returns the token being held.

beneficiary() → address public

Returns the beneficiary that will receive the tokens.

releaseTime() → uint256 public

Returns the time when the tokens are released in seconds since Unix epoch (i.e. Unix timestamp).

release() public

Transfers tokens held by the timelock to the beneficiary. Will only succeed if invoked after the release time.

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