> For the complete documentation index, see [llms.txt](https://docs.fringe.fi/smart-contract-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fringe.fi/smart-contract-documentation/openzeppelin/contracts-upgradeable/token/erc-20/extensions/erc20snapshotupgradeable.md).

# ERC 20 Snapshot Upgradeable

*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. NOTE: 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.*

## Methods

### allowance

```solidity
function allowance(address owner, address spender) external view returns (uint256)
```

*See {IERC20-allowance}.*

#### Parameters

| Name    | Type    | Description |
| ------- | ------- | ----------- |
| owner   | address | undefined   |
| spender | address | undefined   |

#### Returns

| Name | Type    | Description |
| ---- | ------- | ----------- |
| \_0  | uint256 | undefined   |

### approve

```solidity
function approve(address spender, uint256 amount) external nonpayable returns (bool)
```

*See {IERC20-approve}. Requirements: - `spender` cannot be the zero address.*

#### Parameters

| Name    | Type    | Description |
| ------- | ------- | ----------- |
| spender | address | undefined   |
| amount  | uint256 | undefined   |

#### Returns

| Name | Type | Description |
| ---- | ---- | ----------- |
| \_0  | bool | undefined   |

### balanceOf

```solidity
function balanceOf(address account) external view returns (uint256)
```

*See {IERC20-balanceOf}.*

#### Parameters

| Name    | Type    | Description |
| ------- | ------- | ----------- |
| account | address | undefined   |

#### Returns

| Name | Type    | Description |
| ---- | ------- | ----------- |
| \_0  | uint256 | undefined   |

### balanceOfAt

```solidity
function balanceOfAt(address account, uint256 snapshotId) external view returns (uint256)
```

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

#### Parameters

| Name       | Type    | Description |
| ---------- | ------- | ----------- |
| account    | address | undefined   |
| snapshotId | uint256 | undefined   |

#### Returns

| Name | Type    | Description |
| ---- | ------- | ----------- |
| \_0  | uint256 | undefined   |

### decimals

```solidity
function decimals() external view returns (uint8)
```

*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; NOTE: 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}.*

#### Returns

| Name | Type  | Description |
| ---- | ----- | ----------- |
| \_0  | uint8 | undefined   |

### decreaseAllowance

```solidity
function decreaseAllowance(address spender, uint256 subtractedValue) external nonpayable returns (bool)
```

*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`.*

#### Parameters

| Name            | Type    | Description |
| --------------- | ------- | ----------- |
| spender         | address | undefined   |
| subtractedValue | uint256 | undefined   |

#### Returns

| Name | Type | Description |
| ---- | ---- | ----------- |
| \_0  | bool | undefined   |

### increaseAllowance

```solidity
function increaseAllowance(address spender, uint256 addedValue) external nonpayable returns (bool)
```

*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.*

#### Parameters

| Name       | Type    | Description |
| ---------- | ------- | ----------- |
| spender    | address | undefined   |
| addedValue | uint256 | undefined   |

#### Returns

| Name | Type | Description |
| ---- | ---- | ----------- |
| \_0  | bool | undefined   |

### name

```solidity
function name() external view returns (string)
```

*Returns the name of the token.*

#### Returns

| Name | Type   | Description |
| ---- | ------ | ----------- |
| \_0  | string | undefined   |

### symbol

```solidity
function symbol() external view returns (string)
```

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

#### Returns

| Name | Type   | Description |
| ---- | ------ | ----------- |
| \_0  | string | undefined   |

### totalSupply

```solidity
function totalSupply() external view returns (uint256)
```

*See {IERC20-totalSupply}.*

#### Returns

| Name | Type    | Description |
| ---- | ------- | ----------- |
| \_0  | uint256 | undefined   |

### totalSupplyAt

```solidity
function totalSupplyAt(uint256 snapshotId) external view returns (uint256)
```

*Retrieves the total supply at the time `snapshotId` was created.*

#### Parameters

| Name       | Type    | Description |
| ---------- | ------- | ----------- |
| snapshotId | uint256 | undefined   |

#### Returns

| Name | Type    | Description |
| ---- | ------- | ----------- |
| \_0  | uint256 | undefined   |

### transfer

```solidity
function transfer(address recipient, uint256 amount) external nonpayable returns (bool)
```

*See {IERC20-transfer}. Requirements: - `recipient` cannot be the zero address. - the caller must have a balance of at least `amount`.*

#### Parameters

| Name      | Type    | Description |
| --------- | ------- | ----------- |
| recipient | address | undefined   |
| amount    | uint256 | undefined   |

#### Returns

| Name | Type | Description |
| ---- | ---- | ----------- |
| \_0  | bool | undefined   |

### transferFrom

```solidity
function transferFrom(address sender, address recipient, uint256 amount) external nonpayable returns (bool)
```

*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}. 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`.*

#### Parameters

| Name      | Type    | Description |
| --------- | ------- | ----------- |
| sender    | address | undefined   |
| recipient | address | undefined   |
| amount    | uint256 | undefined   |

#### Returns

| Name | Type | Description |
| ---- | ---- | ----------- |
| \_0  | bool | undefined   |

## Events

### Approval

```solidity
event Approval(address indexed owner, address indexed spender, uint256 value)
```

#### Parameters

| Name              | Type    | Description |
| ----------------- | ------- | ----------- |
| owner `indexed`   | address | undefined   |
| spender `indexed` | address | undefined   |
| value             | uint256 | undefined   |

### Snapshot

```solidity
event Snapshot(uint256 id)
```

*Emitted by {\_snapshot} when a snapshot identified by `id` is created.*

#### Parameters

| Name | Type    | Description |
| ---- | ------- | ----------- |
| id   | uint256 | undefined   |

### Transfer

```solidity
event Transfer(address indexed from, address indexed to, uint256 value)
```

#### Parameters

| Name           | Type    | Description |
| -------------- | ------- | ----------- |
| from `indexed` | address | undefined   |
| to `indexed`   | address | undefined   |
| value          | uint256 | undefined   |