1. Writing ERC-20 Smart Contract (2023)

pragma solidity ^0.5.0;

​

/**

* @dev Interface of the ERC20 standard as defined in the EIP. Does not include

* the optional functions; to access them see `ERC20Detailed`.

*/

interface IERC20 {

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);

}

​

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;

}

}

​

/**

* @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`.

*

* *For a detailed writeup see our guide [How to implement supply

* mechanisms](https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226).*

*

* 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 MyERC20 is IERC20 {

using SafeMath for uint256;

​

mapping (address => uint256) private _balances;

​

mapping (address => mapping (address => uint256)) private _allowances;

​

// NOTE Start of https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC20/ERC20Detailed.sol

string private _name;

string private _symbol;

uint8 private _decimals;

​

constructor (string memory name, string memory symbol, uint8 decimals) public {

_name = name;

_symbol = symbol;

_decimals = decimals;

​

_mint(msg.sender, 100000 * 10 ** uint256(decimals)); // CAUTION!

}

​

/**

* @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 that 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;

}

// NOTE End of https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC20/ERC20Detailed.sol

​

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");

​

_balances[account] = _balances[account].sub(value);

_totalSupply = _totalSupply.sub(value);

emit Transfer(account, address(0), value);

}

​

/**

* @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);

}

​

/**

* @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));

}

}