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

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

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

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

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

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

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

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

function name() external view returns (string)

Returns the name of the token.

Returns

Name	Type	Description
_0	string	undefined

symbol

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

function totalSupply() external view returns (uint256)

See {IERC20-totalSupply}.

Returns

Name	Type	Description
_0	uint256	undefined

totalSupplyAt

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

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

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

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

event Snapshot(uint256 id)

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

Parameters

Name	Type	Description
id	uint256	undefined

Transfer

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