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}._

constructor

constructor(string name_, string symbol_) public

_Sets the values for {name} and {symbol}.

The defaut value of {decimals} is 18. To select a different value for {decimals} you should overload it.

All three of these values are immutable: they can only be set once during construction._

name

function name() public view virtual returns (string)

Returns the name of the token.

symbol

function symbol() public view virtual returns (string)

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

decimals

function decimals() public view virtual 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 overloaded;

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}._

totalSupply

function totalSupply() public view virtual returns (uint256)

See {IERC20-totalSupply}.

balanceOf

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

See {IERC20-balanceOf}.

transfer

function transfer(address recipient, uint256 amount) public virtual returns (bool)

_See {IERC20-transfer}.

Requirements:

  • recipient cannot be the zero address.
  • the caller must have a balance of at least amount._

allowance

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

See {IERC20-allowance}.

approve

function approve(address spender, uint256 amount) public virtual returns (bool)

_See {IERC20-approve}.

Requirements:

  • spender cannot be the zero address._

transferFrom

function transferFrom(address sender, address recipient, uint256 amount) public virtual 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._

increaseAllowance

function increaseAllowance(address spender, uint256 addedValue) public virtual 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._

decreaseAllowance

function decreaseAllowance(address spender, uint256 subtractedValue) public virtual 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._

_transfer

function _transfer(address sender, address recipient, uint256 amount) internal virtual

_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._

_mint

function _mint(address account, uint256 amount) internal virtual

_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._

_burn

function _burn(address account, uint256 amount) internal virtual

_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

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

_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._

_beforeTokenTransfer

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

_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]._