cj/t340-2016二级标准（czn2203标准）

#FUNCTIONStotalSupply()balanceOf(account)transfer(recipient, amount)allowance(owner, spender)approve(spender, amount)transferFrom(sender, recipient, amount)    #EVENTSTransfer(from, to, value)Approval(owner, spender, value)

pragma solidity ^0.8.0;
/** * @dev Interface of the ERC20 standard as defined in the EIP. */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);}

pragma solidity ^0.8.0;
import \\\"./IERC20.sol\\\";import \\\"./extensions/IERC20Metadata.sol\\\";import \\\"../../utils/Context.sol\\\";
/** * @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 {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}. */contract ERC20 is Context, IERC20, IERC20Metadata {    mapping (address => uint256) private _balances;
    mapping (address => mapping (address => uint256)) private _allowances;
    uint256 private _totalSupply;
    string private _name;    string private _symbol;
    /**     * @dev 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.     */    constructor (string memory name_, string memory symbol_) {        _name = name_;        _symbol = symbol_;    }
    /**     * @dev Returns the name of the token.     */    function name() public view virtual override returns (string memory) {        return _name;    }
    /**     * @dev Returns the symbol of the token, usually a shorter version of the     * name.     */    function symbol() public view virtual override 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. 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}.     */    function decimals() public view virtual override returns (uint8) {        return 18;    }
    /**     * @dev See {IERC20-totalSupply}.     */    function totalSupply() public view virtual override returns (uint256) {        return _totalSupply;    }
    /**     * @dev See {IERC20-balanceOf}.     */    function balanceOf(address account) public view virtual override 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 virtual override returns (bool) {        _transfer(_msgSender(), recipient, amount);        return true;    }
    /**     * @dev See {IERC20-allowance}.     */    function allowance(address owner, address spender) public view virtual override returns (uint256) {        return _allowances[owner][spender];    }
    /**     * @dev See {IERC20-approve}.     *     * Requirements:     *     * - `spender` cannot be the zero address.     */    function approve(address spender, uint256 amount) public virtual override returns (bool) {        _approve(_msgSender(), spender, amount);        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 `amount`.     * - the caller must have allowance for ``sender``\\\'s tokens of at least     * `amount`.     */    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {        _transfer(sender, recipient, amount);
        uint256 currentAllowance = _allowances[sender][_msgSender()];        require(currentAllowance >= amount, \\\"ERC20: transfer amount exceeds allowance\\\");        _approve(sender, _msgSender(), currentAllowance - 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 virtual returns (bool) {        _approve(_msgSender(), spender, _allowances[_msgSender()][spender] + 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 virtual returns (bool) {        uint256 currentAllowance = _allowances[_msgSender()][spender];        require(currentAllowance >= subtractedValue, \\\"ERC20: decreased allowance below zero\\\");        _approve(_msgSender(), spender, currentAllowance - 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 virtual {        require(sender != address(0), \\\"ERC20: transfer from the zero address\\\");        require(recipient != address(0), \\\"ERC20: transfer to the zero address\\\");
        _beforeTokenTransfer(sender, recipient, amount);
        uint256 senderBalance = _balances[sender];        require(senderBalance >= amount, \\\"ERC20: transfer amount exceeds balance\\\");        _balances[sender] = senderBalance - amount;        _balances[recipient] += 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 virtual {        require(account != address(0), \\\"ERC20: mint to the zero address\\\");
        _beforeTokenTransfer(address(0), account, amount);
        _totalSupply += amount;        _balances[account] += 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 amount) internal virtual {        require(account != address(0), \\\"ERC20: burn from the zero address\\\");
        _beforeTokenTransfer(account, address(0), amount);
        uint256 accountBalance = _balances[account];        require(accountBalance >= amount, \\\"ERC20: burn amount exceeds balance\\\");        _balances[account] = accountBalance - amount;        _totalSupply -= amount;
        emit Transfer(account, address(0), amount);    }
    /**     * @dev 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.     */    function _approve(address owner, address spender, uint256 amount) internal virtual {        require(owner != address(0), \\\"ERC20: approve from the zero address\\\");        require(spender != address(0), \\\"ERC20: approve to the zero address\\\");
        _allowances[owner][spender] = amount;        emit Approval(owner, spender, amount);    }
    /**     * @dev 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].     */    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }}

contract ERC20 is Context, IERC20, IERC20Metadata {

uint256 private _totalSupply;      //发行总量string private _name;          //代币名称string private _symbol;          //代币标识符

constructor (string memory name_, string memory symbol_) {    _name = name_;          //初始化代币名称    _symbol = symbol_;        //初始化代币标识符}

    /**     * @dev Returns the name of the token.     */    function name() public view virtual override returns (string memory) {        return _name;    }
    /**     * @dev Returns the symbol of the token, usually a shorter version of the     * name.     */    function symbol() public view virtual override 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. 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}.     */    function decimals() public view virtual override returns (uint8) {        return 18;    }
    /**     * @dev See {IERC20-totalSupply}.     */    function totalSupply() public view virtual override returns (uint256) {        return _totalSupply;    }
    /**     * @dev See {IERC20-balanceOf}.     */    function balanceOf(address account) public view virtual override 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 virtual override returns (bool) {        _transfer(_msgSender(), recipient, amount);        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 virtual {        require(sender != address(0), \\\"ERC20: transfer from the zero address\\\");        require(recipient != address(0), \\\"ERC20: transfer to the zero address\\\");
        _beforeTokenTransfer(sender, recipient, amount);
        uint256 senderBalance = _balances[sender];        require(senderBalance >= amount, \\\"ERC20: transfer amount exceeds balance\\\");        _balances[sender] = senderBalance - amount;        _balances[recipient] += amount;
        emit Transfer(sender, recipient, amount);    }

    /**     * @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 `amount`.     * - the caller must have allowance for ``sender``\\\'s tokens of at least     * `amount`.     */    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {        _transfer(sender, recipient, amount);
        uint256 currentAllowance = _allowances[sender][_msgSender()];        require(currentAllowance >= amount, \\\"ERC20: transfer amount exceeds allowance\\\");        _approve(sender, _msgSender(), currentAllowance - amount);
        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 virtual {        require(sender != address(0), \\\"ERC20: transfer from the zero address\\\");        require(recipient != address(0), \\\"ERC20: transfer to the zero address\\\");
        _beforeTokenTransfer(sender, recipient, amount);
        uint256 senderBalance = _balances[sender];        require(senderBalance >= amount, \\\"ERC20: transfer amount exceeds balance\\\");        _balances[sender] = senderBalance - amount;        _balances[recipient] += amount;
        emit Transfer(sender, recipient, amount);    }

uint256 currentAllowance = _allowances[sender][_msgSender()];require(currentAllowance >= amount, \\\"ERC20: transfer amount exceeds allowance\\\");

    /**     * @dev 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.     */    function _approve(address owner, address spender, uint256 amount) internal virtual {        require(owner != address(0), \\\"ERC20: approve from the zero address\\\");        require(spender != address(0), \\\"ERC20: approve to the zero address\\\");
        _allowances[owner][spender] = amount;        emit Approval(owner, spender, amount);    }

    /**     * @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 virtual returns (bool) {        _approve(_msgSender(), spender, _allowances[_msgSender()][spender] + 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 virtual returns (bool) {        uint256 currentAllowance = _allowances[_msgSender()][spender];        require(currentAllowance >= subtractedValue, \\\"ERC20: decreased allowance below zero\\\");        _approve(_msgSender(), spender, currentAllowance - subtractedValue);
        return true;    }

    /** @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 virtual {        require(account != address(0), \\\"ERC20: mint to the zero address\\\");
        _beforeTokenTransfer(address(0), account, amount);
        _totalSupply += amount;        _balances[account] += 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 amount) internal virtual {        require(account != address(0), \\\"ERC20: burn from the zero address\\\");
        _beforeTokenTransfer(account, address(0), amount);
        uint256 accountBalance = _balances[account];        require(accountBalance >= amount, \\\"ERC20: burn amount exceeds balance\\\");        _balances[account] = accountBalance - amount;        _totalSupply -= amount;
        emit Transfer(account, address(0), amount);    }