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ERC20.sol

View Source: @openzeppelin/contracts/token/ERC20/ERC20.sol

↗ Extends: Context, IERC20 ↘ Derived Contracts: NueToken

ERC20

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}.

Contract Members

Constants & Variables

mapping(address => uint256) private _balances;
mapping(address => mapping(address => uint256)) private _allowances;
uint256 private _totalSupply;
string private _name;
string private _symbol;
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.

function (string name_, string symbol_) public nonpayable

Arguments

Name Type Description
name_ string
symbol_ string
Source Code 
constructor (string memory name_, string memory symbol_) public {
        _name = name_;
        _symbol = symbol_;
        _decimals = 18;
    }

name

Returns the name of the token.

function name() public view
returns(string)
Source Code 
function name() public view virtual returns (string memory) {
        return _name;
    }

symbol

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

function symbol() public view
returns(string)
Source Code 
function symbol() public view virtual returns (string memory) {
        return _symbol;
    }

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)
Source Code 
function decimals() public view virtual returns (uint8) {
        return _decimals;
    }

totalSupply

See {IERC20-totalSupply}.

function totalSupply() public view
returns(uint256)
Source Code 
function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }

balanceOf

See {IERC20-balanceOf}.

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

Arguments

Name Type Description
account address
Source Code 
function balanceOf(address account) public view virtual override returns (uint256) {
        return _balances[account];
    }

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 nonpayable
returns(bool)

Arguments

Name Type Description
recipient address
amount uint256
Source Code 
function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

allowance

See {IERC20-allowance}.

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

Arguments

Name Type Description
owner address
spender address
Source Code 
function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

approve

See {IERC20-approve}. Requirements:

  • spender cannot be the zero address.
function approve(address spender, uint256 amount) public nonpayable
returns(bool)

Arguments

Name Type Description
spender address
amount uint256
Source Code 
function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

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 nonpayable
returns(bool)

Arguments

Name Type Description
sender address
recipient address
amount uint256
Source Code 
function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

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 nonpayable
returns(bool)

Arguments

Name Type Description
spender address
addedValue uint256
Source Code 
function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

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 nonpayable
returns(bool)

Arguments

Name Type Description
spender address
subtractedValue uint256
Source Code 
function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

_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 nonpayable

Arguments

Name Type Description
sender address
recipient address
amount uint256
Source Code 
function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

_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 nonpayable

Arguments

Name Type Description
account address
amount uint256
Source Code 
function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

_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 nonpayable

Arguments

Name Type Description
account address
amount uint256
Source Code 
function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

_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 nonpayable

Arguments

Name Type Description
owner address
spender address
amount uint256
Source Code 
function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

_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 nonpayable

Arguments

Name Type Description
decimals_ uint8
Source Code 
function _setupDecimals(uint8 decimals_) internal virtual {
        _decimals = decimals_;
    }

_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 nonpayable

Arguments

Name Type Description
from address
to address
amount uint256
Source Code 
function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }

Contracts