The ERC-20 standard, while originally proposed for Ethereum, is also applicable to TEN's Layer 2. It provides an API for tokens within Smart Contracts and offers functionalities to:
Transfer tokens between accounts on TEN
Retrieve the current token balance of an account on TEN
Determine the total supply of the token on the TEN network
Approve third-party accounts to spend a specific amount of tokens on TEN
A Smart Contract on TEN that implements the following methods and events adheres to the ERC-20 standard:
Methods:
name(): Returns the name of the token.
symbol(): Returns the symbol of the token.
decimals(): Returns the number of decimals the token uses.
totalSupply(): Returns the total token supply.
balanceOf(address _owner): Returns the token balance of the specified address.
transfer(address _to, uint256 _value): Transfers tokens to a specified address.
transferFrom(address _from, address _to, uint256 _value): Transfers tokens from one address to another.
approve(address _spender, uint256 _value): Approves a third-party account to spend a specified amount of tokens.
allowance(address _owner, address _spender): Returns the amount of tokens approved by an owner for a spender.
Events:
Transfer(address indexed _from, address indexed _to, uint256 _value): Triggered when tokens are transferred.
Approval(address indexed _owner, address indexed _spender, uint256 _value): Triggered when approve function is called.
pragma solidity ^0.4.24;
import "./IERC20.sol";
import "../../math/SafeMath.sol";
/**
* @title Standard ERC20 token
*
* @dev Implementation of the basic standard token.
* https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
* Originally based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
*/
contract ERC20 is IERC20 {
using SafeMath for uint256;
mapping (address => uint256) private _balances;
mapping (address => mapping (address => uint256)) private _allowed;
uint256 private _totalSupply;
/**
* @dev Total number of tokens in existence
*/
function totalSupply() public view returns (uint256) {
return _totalSupply;
}
/**
* @dev Gets the balance of the specified address.
* @param owner The address to query the balance of.
* @return An uint256 representing the amount owned by the passed address.
*/
function balanceOf(address owner) public view returns (uint256) {
return _balances[owner];
}
/**
* @dev Function to check the amount of tokens that an owner allowed to a spender.
* @param owner address The address which owns the funds.
* @param spender address The address which will spend the funds.
* @return A uint256 specifying the amount of tokens still available for the spender.
*/
function allowance(
address owner,
address spender
)
public
view
returns (uint256)
{
return _allowed[owner][spender];
}
/**
* @dev Transfer token for a specified address
* @param to The address to transfer to.
* @param value The amount to be transferred.
*/
function transfer(address to, uint256 value) public returns (bool) {
require(value <= _balances[msg.sender]);
require(to != address(0));
_balances[msg.sender] = _balances[msg.sender].sub(value);
_balances[to] = _balances[to].add(value);
emit Transfer(msg.sender, to, value);
return true;
}
/**
* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
* 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
* @param spender The address which will spend the funds.
* @param value The amount of tokens to be spent.
*/
function approve(address spender, uint256 value) public returns (bool) {
require(spender != address(0));
_allowed[msg.sender][spender] = value;
emit Approval(msg.sender, spender, value);
return true;
}
/**
* @dev Transfer tokens from one address to another
* @param from address The address which you want to send tokens from
* @param to address The address which you want to transfer to
* @param value uint256 the amount of tokens to be transferred
*/
function transferFrom(
address from,
address to,
uint256 value
)
public
returns (bool)
{
require(value <= _balances[from]);
require(value <= _allowed[from][msg.sender]);
require(to != address(0));
_balances[from] = _balances[from].sub(value);
_balances[to] = _balances[to].add(value);
_allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
emit Transfer(from, to, value);
return true;
}
/**
* @dev Increase the amount of tokens that an owner allowed to a spender.
* approve should be called when allowed_[_spender] == 0. To increment
* allowed value is better to use this function to avoid 2 calls (and wait until
* the first transaction is mined)
* From MonolithDAO Token.sol
* @param spender The address which will spend the funds.
* @param addedValue The amount of tokens to increase the allowance by.
*/
function increaseAllowance(
address spender,
uint256 addedValue
)
public
returns (bool)
{
require(spender != address(0));
_allowed[msg.sender][spender] = (
_allowed[msg.sender][spender].add(addedValue));
emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
return true;
}
/**
* @dev Decrease the amount of tokens that an owner allowed to a spender.
* approve should be called when allowed_[_spender] == 0. To decrement
* allowed value is better to use this function to avoid 2 calls (and wait until
* the first transaction is mined)
* From MonolithDAO Token.sol
* @param spender The address which will spend the funds.
* @param subtractedValue The amount of tokens to decrease the allowance by.
*/
function decreaseAllowance(
address spender,
uint256 subtractedValue
)
public
returns (bool)
{
require(spender != address(0));
_allowed[msg.sender][spender] = (
_allowed[msg.sender][spender].sub(subtractedValue));
emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
return true;
}
/**
* @dev Internal function that mints an amount of the token and assigns it to
* an account. This encapsulates the modification of balances such that the
* proper events are emitted.
* @param account The account that will receive the created tokens.
* @param amount The amount that will be created.
*/
function _mint(address account, uint256 amount) internal {
require(account != 0);
_totalSupply = _totalSupply.add(amount);
_balances[account] = _balances[account].add(amount);
emit Transfer(address(0), account, amount);
}
/**
* @dev Internal function that burns an amount of the token of a given
* account.
* @param account The account whose tokens will be burnt.
* @param amount The amount that will be burnt.
*/
function _burn(address account, uint256 amount) internal {
require(account != 0);
require(amount <= _balances[account]);
_totalSupply = _totalSupply.sub(amount);
_balances[account] = _balances[account].sub(amount);
emit Transfer(account, address(0), amount);
}
/**
* @dev Internal function that burns an amount of the token of a given
* account, deducting from the sender's allowance for said account. Uses the
* internal burn function.
* @param account The account whose tokens will be burnt.
* @param amount The amount that will be burnt.
*/
function _burnFrom(address account, uint256 amount) internal {
require(amount <= _allowed[account][msg.sender]);
// Should https://github.com/OpenZeppelin/zeppelin-solidity/issues/707 be accepted,
// this function needs to emit an event with the updated approval.
_allowed[account][msg.sender] = _allowed[account][msg.sender].sub(
amount);
_burn(account, amount);
}
}
