A standard ERC20 contract supports the following interface:
function totalSupply() external view returns (uint256);
function balanceOf(address account) external view returns (uint256);
function transfer(address recipient, uint256 amount) external returns (bool);
function allowance(address owner, address spender) external view returns (uint256);
function approve(address spender, uint256 amount) external returns (bool);
function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
event Transfer(address indexed from, address indexed to, uint256 value);
event Approval(address indexed owner, address indexed spender, uint256 value);
In this tutorial, a contract based on the implementation of OpenZeppelin will be used, with the addition of token transfer functionality to the creator account at the time the contract is created. The original contract can be found here - ERC20.sol
For ease of use, contract dependencies are collected in one file. As a result, the contract is as follows:
pragma solidity 0.4.23;
/**
* @dev Standard math utilities missing in the Solidity language.
*/
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-solidity/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.
*
* > 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 = 10000;
constructor() public {
_balances[msg.sender] = 10000;
emit Transfer(address(0), msg.sender, 10000);
}
/**
* @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 Sets `amount` as the allowance of `spender` over the `owner`s 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);
}
}
As with Ethereum, the following artifacts are required to work with a contract within Echo:
- contract ByteCode
- function hashes
- ABI (for using by libraries or wallets)
One way to get these artifacts is to use https://remix.ethereum.org/.
Please note that at the moment Echo supports the Solidity version <= 0.4.23, therefore, when compiling a contract, solc version <= 0.4.23 should be chosen. The result of the compilation is as follows:
{% tabs %} {% tab title="ByteCode" %}
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{% endtab %}
{% tab title="Hashes" %}
"39509351": "increaseAllowance(address,uint256)",
"dd62ed3e": "allowance(address,address)",
"095ea7b3": "approve(address,uint256)",
"70a08231": "balanceOf(address)",
"a457c2d7": "decreaseAllowance(address,uint256)",
"18160ddd": "totalSupply()",
"a9059cbb": "transfer(address,uint256)",
"23b872dd": "transferFrom(address,address,uint256)"{% endtab %}
{% tab title="ABI" %}
[
{
"constant": false,
"inputs": [
{
"name": "spender",
"type": "address"
},
{
"name": "value",
"type": "uint256"
}
],
"name": "approve",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": true,
"inputs": [],
"name": "totalSupply",
"outputs": [
{
"name": "",
"type": "uint256"
}
],
"payable": false,
"stateMutability": "view",
"type": "function"
},
{
"constant": false,
"inputs": [
{
"name": "sender",
"type": "address"
},
{
"name": "recipient",
"type": "address"
},
{
"name": "amount",
"type": "uint256"
}
],
"name": "transferFrom",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": false,
"inputs": [
{
"name": "spender",
"type": "address"
},
{
"name": "addedValue",
"type": "uint256"
}
],
"name": "increaseAllowance",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": true,
"inputs": [
{
"name": "account",
"type": "address"
}
],
"name": "balanceOf",
"outputs": [
{
"name": "",
"type": "uint256"
}
],
"payable": false,
"stateMutability": "view",
"type": "function"
},
{
"constant": false,
"inputs": [
{
"name": "spender",
"type": "address"
},
{
"name": "subtractedValue",
"type": "uint256"
}
],
"name": "decreaseAllowance",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": false,
"inputs": [
{
"name": "recipient",
"type": "address"
},
{
"name": "amount",
"type": "uint256"
}
],
"name": "transfer",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": true,
"inputs": [
{
"name": "owner",
"type": "address"
},
{
"name": "spender",
"type": "address"
}
],
"name": "allowance",
"outputs": [
{
"name": "",
"type": "uint256"
}
],
"payable": false,
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"payable": false,
"stateMutability": "nonpayable",
"type": "constructor"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"name": "from",
"type": "address"
},
{
"indexed": true,
"name": "to",
"type": "address"
},
{
"indexed": false,
"name": "value",
"type": "uint256"
}
],
"name": "Transfer",
"type": "event"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"name": "owner",
"type": "address"
},
{
"indexed": true,
"name": "spender",
"type": "address"
},
{
"indexed": false,
"name": "value",
"type": "uint256"
}
],
"name": "Approval",
"type": "event"
}
]{% endtab %} {% endtabs %}
Also solc can be used for compilation. At first, you should save the contract to the file erc20.sol and then you can use the following line to achieve a similar result:
$ ./solc --bin --hashes erc20.sol
======= piggy.sol:PiggyBank =======
Binary:
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Function signatures:
"39509351": "increaseAllowance(address,uint256)",
"dd62ed3e": "allowance(address,address)",
"095ea7b3": "approve(address,uint256)",
"70a08231": "balanceOf(address)",
"a457c2d7": "decreaseAllowance(address,uint256)",
"18160ddd": "totalSupply()",
"a9059cbb": "transfer(address,uint256)",
"23b872dd": "transferFrom(address,address,uint256)"This approach will be useful when you need to compile a contract that uses advanced Echo Solidity functionality. You can read more about this in the corresponding section.
Below are several approaches to working with a contract: