Contract Example
pragma solidity ^0.5.0;
/**
@dev 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 {ERC20Mintable}.
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}. / library SafeMath { /*
@dev Returns the addition of two unsigned integers, reverting on
overflow.
Counterpart to Solidity's + operator.
Requirements:
Addition cannot overflow. */ function add(uint256 a, uint256 b) internal pure returns (uint256) { uint256 c = a + b; require(c >= a, "SafeMath: addition overflow");
return c; }
/**
@dev Returns the subtraction of two unsigned integers, reverting on
overflow (when the result is negative).
Counterpart to Solidity's - operator.
Requirements:
Subtraction cannot overflow. */ function sub(uint256 a, uint256 b) internal pure returns (uint256) { require(b <= a, "SafeMath: subtraction overflow"); uint256 c = a - b;
return c; }
/**
@dev Returns the multiplication of two unsigned integers, reverting on
overflow.
Counterpart to Solidity's * operator.
Requirements:
Multiplication cannot overflow. */ function mul(uint256 a, uint256 b) internal pure returns (uint256) { // Gas optimization: this is cheaper than requiring 'a' not being zero, but the // benefit is lost if 'b' is also tested. // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522 if (a == 0) { return 0; }
uint256 c = a * b; require(c / a == b, "SafeMath: multiplication overflow");
return c; }
/**
@dev Returns the integer division of two unsigned integers. Reverts on
division by zero. The result is rounded towards zero.
Counterpart to Solidity's / operator. Note: this function uses a
revert opcode (which leaves remaining gas untouched) while Solidity
uses an invalid opcode to revert (consuming all remaining gas).
Requirements:
The divisor cannot be zero. */ function div(uint256 a, uint256 b) internal pure returns (uint256) { // Solidity only automatically asserts when dividing by 0 require(b > 0, "SafeMath: division by zero"); uint256 c = a / b; // assert(a == b * c + a % b); // There is no case in which this doesn't hold
return c; }
/**
@dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
Reverts when dividing by zero.
Counterpart to Solidity's % operator. This function uses a revert
opcode (which leaves remaining gas untouched) while Solidity uses an
invalid opcode to revert (consuming all remaining gas).
Requirements:
The divisor cannot be zero. / function mod(uint256 a, uint256 b) internal pure returns (uint256) { require(b != 0, "SafeMath: modulo by zero"); return a % b; } } interface IERC20 { /*
@dev Returns the amount of tokens in existence. */ function totalSupply() external view returns (uint256);
/**
@dev Returns the amount of tokens owned by account. */ function balanceOf(address account) external view returns (uint256);
/**
@dev Moves amount tokens from the caller's account to recipient.
Returns a boolean value indicating whether the operation succeeded.
Emits a {Transfer} event. */ function transfer(address recipient, uint256 amount) external returns (bool);
/**
@dev Returns the remaining number of tokens that spender will be
allowed to spend on behalf of owner through {transferFrom}. This is
zero by default.
This value changes when {approve} or {transferFrom} are called. */ function allowance(address owner, address spender) external view returns (uint256);
/**
@dev Sets amount as the allowance of spender over the caller's tokens.
Returns a boolean value indicating whether the operation succeeded.
IMPORTANT: Beware that changing an allowance with this method brings the risk
that someone may use both the old and the new allowance by unfortunate
transaction ordering. One possible solution to mitigate this race
condition is to first reduce the spender's allowance to 0 and set the
desired value afterwards:
https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
Emits an {Approval} event. */ function approve(address spender, uint256 amount) external returns (bool);
/**
@dev Moves amount tokens from sender to recipient using the
allowance mechanism. amount is then deducted from the caller's
allowance.
Returns a boolean value indicating whether the operation succeeded.
Emits a {Transfer} event. */ function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
/**
@dev Emitted when value tokens are moved from one account (from) to
another (to).
Note that value may be zero. */ event Transfer(address indexed from, address indexed to, uint256 value);
/**
@dev Emitted when the allowance of a spender for an owner is set by
a call to {approve}. value is the new allowance. */ event Approval(address indexed owner, address indexed spender, uint256 value); } contract ERC20 is IERC20 { using SafeMath for uint256;
mapping (address => uint256) private _balances;
mapping (address => mapping (address => uint256)) private _allowances;
uint256 private _totalSupply;
/**
@dev See {IERC20-totalSupply}. */ function totalSupply() public view returns (uint256) { return _totalSupply; }
/**
@dev See {IERC20-balanceOf}. */ function balanceOf(address account) public view returns (uint256) { return _balances[account]; }
/**
@dev 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 returns (bool) { _transfer(msg.sender, recipient, amount); return true; }
/**
@dev See {IERC20-allowance}. */ function allowance(address owner, address spender) public view returns (uint256) { return _allowances[owner][spender]; }
/**
@dev See {IERC20-approve}.
Requirements:
spender cannot be the zero address. */ function approve(address spender, uint256 value) public returns (bool) { _approve(msg.sender, spender, value); return true; }
/**
@dev 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 value.
the caller must have allowance for sender's tokens of at least
amount. */ function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) { _transfer(sender, recipient, amount); _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount)); return true; }
/**
@dev 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 returns (bool) { _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue)); return true; }
/**
@dev 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 returns (bool) { _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue)); return true; }
/**
@dev 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 { require(sender != address(0), "ERC20: transfer from the zero address"); require(recipient != address(0), "ERC20: transfer to the zero address");
_balances[sender] = _balances[sender].sub(amount); _balances[recipient] = _balances[recipient].add(amount); emit Transfer(sender, recipient, amount); }
/** @dev 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 { require(account != address(0), "ERC20: mint to the zero address");
_totalSupply = _totalSupply.add(amount); _balances[account] = _balances[account].add(amount); emit Transfer(address(0), account, amount); }
/**
@dev 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 value) internal { require(account != address(0), "ERC20: burn from the zero address");
_totalSupply = _totalSupply.sub(value); _balances[account] = _balances[account].sub(value); emit Transfer(account, address(0), value); }
/**
@dev Sets amount as the allowance of spender over the owners tokens.
This is 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 value) internal { require(owner != address(0), "ERC20: approve from the zero address"); require(spender != address(0), "ERC20: approve to the zero address");
_allowances[owner][spender] = value; emit Approval(owner, spender, value); }
/**
@dev Destoys amount tokens from account.amount is then deducted
from the caller's allowance.
See {_burn} and {_approve}. */ function _burnFrom(address account, uint256 amount) internal { _burn(account, amount); _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount)); } } contract ERC20Detailed is IERC20 { string private _name; string private _symbol; uint8 private _decimals;
/**
@dev Sets the values for name, symbol, and decimals. All three of
these values are immutable: they can only be set once during
construction. */ constructor (string memory name, string memory symbol, uint8 decimals) public { _name = name; _symbol = symbol; _decimals = decimals; }
/**
@dev Returns the name of the token. */ function name() public view returns (string memory) { return _name; }
/**
@dev Returns the symbol of the token, usually a shorter version of the
name. */ function symbol() public view returns (string memory) { return _symbol; }
/**
@dev 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.
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) { return _decimals; } } contract TokenWRC20 is ERC20, ERC20Detailed {
/**
@dev Constructor that gives msg.sender all of existing tokens. _mint(msg.sender, 10000000000 * (10 ** uint256(decimals()))); */ constructor () public ERC20Detailed("YourTokenName", "YTN", 18) {
} }Last updated