本文给出 0.6.0 版本的标准ERC20智能合约（源码来自https://github.com/OpenZeppelin/openzeppelin-contracts），供大家参考。
1.标准ERC20合约源码：
该部分由4部分组成；
1.1 SafeMath.sol
主要处理算术运算中的溢出问题，达到更安全的计算目的；使用该库可以消除没有检测的计算类错误；因此建议始终使用它；
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
/**
* @dev Wrappers over Solidity's arithmetic operations with added overflow
* checks.
*
* Arithmetic operations in Solidity wrap on overflow. This can easily result
* in bugs, because programmers usually assume that an overflow raises an
* error, which is the standard behavior in high level programming languages.
* `SafeMath` restores this intuition by reverting the transaction when an
* operation overflows.
*
* Using this library instead of the unchecked operati