# ERC 20 Codes

By [journeywest.eth](https://paragraph.com/@journeywest-eth) · 2022-06-09

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// SPDX-License-Identifier: MIT // OpenZeppelin Contracts (last updated v4.6.0) (token/ERC20/ERC20.sol)

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](https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226)\[How
    
*   to implement supply mechanisms\].
    
*     
    
*   We have followed general OpenZeppelin Contracts guidelines: functions revert
    
*   instead 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 default value of {decimals} is 18. To select a different value for
        
    *   {decimals} you should overload it.
        
    *     
        
    *   All two 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
        
    *   overridden;
        
    *     
        
    *   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:
        
    *     
        
    *   `to` cannot be the zero address.
        
    *   the caller must have a balance of at least `amount`. \*/ function transfer(address to, uint256 amount) public virtual override returns (bool) { address owner = \_msgSender(); \_transfer(owner, to, 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}.
        
    *     
        
    *   NOTE: If `amount` is the maximum `uint256`, the allowance is not updated on
        
    *   `transferFrom`. This is semantically equivalent to an infinite approval.
        
    *     
        
    *   Requirements:
        
    *     
        
    *   `spender` cannot be the zero address. \*/ function approve(address spender, uint256 amount) public virtual override returns (bool) { address owner = \_msgSender(); \_approve(owner, 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}.
        
    *     
        
    *   NOTE: Does not update the allowance if the current allowance
        
    *   is the maximum `uint256`.
        
    *     
        
    *   Requirements:
        
    *     
        
    *   `from` and `to` cannot be the zero address.
        
    *   `from` must have a balance of at least `amount`.
        
    *   the caller must have allowance for `from`'s tokens of at least
        
    *   `amount`. \*/ function transferFrom( address from, address to, uint256 amount ) public virtual override returns (bool) { address spender = \_msgSender(); \_spendAllowance(from, spender, amount); \_transfer(from, to, 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) { address owner = \_msgSender(); \_approve(owner, spender, allowance(owner, 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) { address owner = \_msgSender(); uint256 currentAllowance = allowance(owner, spender); require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero"); unchecked { \_approve(owner, spender, currentAllowance - subtractedValue); }
        
        return true; }
        
    
    /\*\*
    
    *   @dev Moves `amount` of tokens from `sender` to `recipient`.
        
    *     
        
    *   This 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:
        
    *     
        
    *   `from` cannot be the zero address.
        
    *   `to` cannot be the zero address.
        
    *   `from` must have a balance of at least `amount`. \*/ function \_transfer( address from, address to, uint256 amount ) internal virtual { require(from != address(0), "ERC20: transfer from the zero address"); require(to != address(0), "ERC20: transfer to the zero address");
        
        \_beforeTokenTransfer(from, to, amount);
        
        uint256 fromBalance = \_balances\[from\]; require(fromBalance >= amount, "ERC20: transfer amount exceeds balance"); unchecked { \_balances\[from\] = fromBalance - amount; } \_balances\[to\] += amount;
        
        emit Transfer(from, to, amount);
        
        \_afterTokenTransfer(from, to, 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:
        
    *     
        
    *   `account` 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);
        
        \_afterTokenTransfer(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"); unchecked { \_balances\[account\] = accountBalance - amount; } \_totalSupply -= amount;
        
        emit Transfer(account, address(0), amount);
        
        \_afterTokenTransfer(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 Updates `owner` s allowance for `spender` based on spent `amount`.
        
    *     
        
    *   Does not update the allowance amount in case of infinite allowance.
        
    *   Revert if not enough allowance is available.
        
    *     
        
    *   Might emit an {Approval} event. \*/ function \_spendAllowance( address owner, address spender, uint256 amount ) internal virtual { uint256 currentAllowance = allowance(owner, spender); if (currentAllowance != type(uint256).max) { require(currentAllowance >= amount, "ERC20: insufficient allowance"); unchecked { \_approve(owner, spender, currentAllowance - 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 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 {}
        
    
    /\*\*
    
    *   @dev Hook that is called after any transfer of tokens. This includes
        
    *   minting and burning.
        
    *     
        
    *   Calling conditions:
        
    *     
        
    *   when `from` and `to` are both non-zero, `amount` of `from`'s tokens
        
    *   has been transferred to `to`.
        
    *   when `from` is zero, `amount` tokens have been minted for `to`.
        
    *   when `to` is zero, `amount` of `from`'s tokens have been 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 \_afterTokenTransfer( address from, address to, uint256 amount ) internal virtual {} }

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*Originally published on [journeywest.eth](https://paragraph.com/@journeywest-eth/erc-20-codes)*