代币标准的目的

代币是以太坊这类通用编程区块链之上最显著和有用的应用形态。

• 代币标准（ERC20、ERC721）是实现的最小单元
• 使用代币标准，可完成合约之间的互操作性（多个合约去实现ERC20）
• 安全

ERC20代币标准

最小单元

6个函数、2个事件

// 获取系统金额发行量
function totalSupply() virtual public view returns (uint totalSupply);
// 获取_owner余额
function balanceOf(address _owner) virtual public view returns (uint balance);
// 调用者(msg.sender)向 _t0 转账 _value
function transfer(address _to, uint256 _value) virtual public returns (bool success);
// 下面两个函数通常一起使用
// 2、授权后，被授权的人调用此函数，相当于手动取钱！ 与transfer的区别在于 一个是甲方直接给你钱，一个是甲方授权，乙方自己取钱。
function transferFrom(address _from, address _to, uint256 _value) virtual public returns (bool success);
// 1、用于授权，msg.sender 授权给 _spender 可以动用 _value
function approve(address _spender, uint256 _value) virtual public returns (bool success);
// 查看_owner 授权给 _spender 的余额数量
function allowance(address _owner, address _spender) virtual public view returns (uint remaining);

// 转账事件，每次发生转账交易后触发事件让别人知道
event Transfer(address indexed _from, address indexed _to, uint256 _value);
// 授权事件，每次发生一个地址给另一个地址授权后触发事件让别人知道
event Approval(address indexed _owner, address indexed _spender, uint256 _value);
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可选单元

代币的名称

function name() virtual public view returns (string calldata);
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代币的代号、标识，通常为字母缩写

function symbol() virtual public view returns (string calldata);
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返回令牌使用的小数位数 - 例如"8"，意味着将令牌量除以"100000000"以获取其用户的表示形式。

function decimals() virtual public view returns (uint8);
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solidity 0.6.0版本以后的不同点

1、合约contract 关键字之前需要加上abstract修饰

2、在抽象的函数内，添加virtual关键字

3、在继承函数时，需要对继承的函数名中加上override修饰符

实战代码展示

ERC20.sol合约：

pragma solidity ^0.6.0;

abstract contract ERC20 {
    // function name() public view returns (string)
    // function symbol() public view returns (string)
    // function decimals() public view returns (uint8)
    function totalSupply() virtual public view returns (uint totalSupply);
    function balanceOf(address _owner) virtual public view returns (uint balance);
    function transfer(address _to, uint256 _value) virtual public returns (bool success);
    function transferFrom(address _from, address _to, uint256 _value) virtual public returns (bool success);
    function approve(address _spender, uint256 _value) virtual public returns (bool success);
    function allowance(address _owner, address _spender) virtual public view returns (uint remaining);

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

}
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pcCoin.sol合约 继承自 ERC20

pragma solidity ^0.6.0;

import "./ERC20.sol";

contract pcCoin is ERC20{
    uint256 _totalSupply; // 总供应量
    address public  _owner; // 部署合约的人
    mapping(address=>uint256) _balances; // address的余额
    mapping(address=>mapping(address=>uint256)) _approves; // a授权给b多少余额


    // 转账事件，转过之后让人能看到
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    // 授权事件，授权之后让人能看到
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    constructor(uint256 totalSupply) public {
        _owner = msg.sender; // _owner初始化为部署合约的人
        _totalSupply = totalSupply;
    }

    // 设置权限
    modifier onlyOwner(){
        require(msg.sender == _owner);
        _;
    }

    //给指定账户投钱，只有部署合约的人有这个权限
    function airDrop(address _to, uint256 _value) onlyOwner public {
        require(_to != address(0) && _value > 0);
        _balances[_to] = _value;
        _totalSupply += _value;
    }

    // 返回总供应量
    function totalSupply() override public view returns(uint totalSupply){
        totalSupply = _totalSupply;
        return totalSupply;
    }

    function balanceOf(address _owner) override public view returns(uint balance){
        require(_owner != address(0), "owner should not be empty! ");
        return _balances[_owner];
    }

    function transfer(address _to, uint256 _value) override public returns (bool success){
        require(_to != address(0), "_to should not be empty !");
        require(_balances[msg.sender] >= _value && _value>0, "value should be valid !!");
        _balances[msg.sender] -= _value;
        _balances[_to] += _value;
        success = true;
        emit Transfer(msg.sender, _to, _value);
        return success;
    }

    // 被授权者来调用此函数，可理解为自己取钱，_from是授权的人
    function transferFrom(address _from, address _to, uint256 _value) override public returns (bool success){
        require(_from != address(0) && _to != address(0));
        require(_approves[_from][msg.sender] >= _value); // _from 授权给 msg.sender 的钱必须大于 _value
        _approves[_from][msg.sender] -= _value; // 取了钱后，_from 授权给 msg.sender 的钱就要减少
        _balances[_to] += _value;
        success = true;
        emit Transfer(_from, _to, _value);
        return success;
    }

    // 调用此函数，意为调用者（msg.sender）授权给_spender多少可动的钱
    function approve(address _spender, uint256 _value) override public returns (bool success){
        require(_spender != address(0));
        require(_balances[msg.sender] >= _value && _value > 0);
        _approves[msg.sender][_spender] += _value; // 授权
        _balances[msg.sender] -= _value; 
        success = true;
        emit Approval(msg.sender, _spender, _value);
        return success;
    }

    function allowance(address _owner, address _spender) override public view returns (uint remaining){
        return _approves[_owner][_spender];
    }

}
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部署到区块链网络（我部署的是私有链ganache网络）

下面可以调函数