ERC20

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Inherits: IERC20

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}. TIP: For a detailed writeup see our guide https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How to implement supply mechanisms]. We have followed general OpenZeppelin guidelines: functions revert instead of 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}.

State Variables

_balances

mapping(address => uint256) private _balances;

_allowances

mapping(address => mapping(address => uint256)) private _allowances;

_totalSupply

uint256 private _totalSupply;

_name

string private _name;

_symbol

string private _symbol;

_decimals

uint8 private _decimals;

Functions

constructor

Sets the values for name and {symbol}, initializes {decimals} with a default value of 18. To select a different value for {decimals}, use {_setupDecimals}. All three of these values are immutable: they can only be set once during construction.

constructor(string memory name, string memory symbol);

name

Returns the name of the token.

function name() public view returns (string memory);

symbol

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

function symbol() public view returns (string memory);

decimals

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 {_setupDecimals} is called. 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}.

function decimals() public view returns (uint8);

totalSupply

See IERC20-totalSupply.

function totalSupply() public view override returns (uint256);

balanceOf

See IERC20-balanceOf.

function balanceOf(address account) public view override returns (uint256);

transfer

*See IERC20-transfer. Requirements:

  • recipient cannot be the zero address.
  • the caller must have a balance of at least amount.*
function transfer(address recipient, uint256 amount) public virtual override returns (bool);

allowance

See IERC20-allowance.

function allowance(address owner, address spender) public view virtual override returns (uint256);

approve

*See IERC20-approve. Requirements:

  • spender cannot be the zero address.*
function approve(address spender, uint256 amount) public virtual override returns (bool);

transferFrom

*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.*
function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool);

increaseAllowance

*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.*
function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool);

decreaseAllowance

*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.*
function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool);

_transfer

*Moves tokens amount from sender to recipient. This is 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.*
function _transfer(address sender, address recipient, uint256 amount) internal virtual;

_mint

*Creates amount tokens and assigns them to account, increasing the total supply. Emits a {Transfer} event with from set to the zero address. Requirements

  • to cannot be the zero address.*
function _mint(address account, uint256 amount) internal virtual;

_burn

*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.*
function _burn(address account, uint256 amount) internal virtual;

_approve

*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.*
function _approve(address owner, address spender, uint256 amount) internal virtual;

_setupDecimals

Sets decimals to a value other than the default one of 18. WARNING: This function should only be called from the constructor. Most applications that interact with token contracts will not expect {decimals} to ever change, and may work incorrectly if it does.

function _setupDecimals(uint8 decimals_) internal;

_beforeTokenTransfer

*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 to 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 xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].*
function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual;