根據(jù)ERC-20標(biāo)準(zhǔn)定義接口

pragma solidity ^0.4.0;

contract ERC20Interface {
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    function name() public view returns (string name);
    function symbol() public view returns (string symbol);
    function decimals() public view returns (uint8 decimals);
    function totalSupply() public view returns (uint256 totalSupply);
    function balanceOf(address _owner) public view returns (uint256 balance);
    function transfer(address _to, uint256 _value) public returns (bool success);
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
    function approve(address _spender, uint256 _value) public returns (bool success);
    function allowance(address _owner, address _spender) public view returns (uint256 remaining);
}

代碼實現(xiàn)一個ERC-20代幣合約

pragma solidity ^0.4.0;

import "./ERC20Interface.sol";

contract ERC20 is ERC20Interface{
    string private _name = "MyToken";
    string private _symbol = "MYT";
    uint8 private _decimals = 3;
    uint256 private _totalSupply = 10 * (10 ** uint256(_decimals));
    mapping(address=>uint256) private _balances;
    mapping(address=>mapping(address=>uint256)) private _allowances;

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
    function ERC20(){
        _balances[msg.sender] = _totalSupply;
    }

    function name() public view returns (string name){
        name = _name;
    }

    function symbol() public view returns (string symbol){
        symbol = _symbol;
    }

    function decimals() public view returns (uint8 decimals){
        decimals = _decimals;
    }

    function totalSupply() public view returns (uint256 totalSupply){
        totalSupply = _totalSupply;
    }

    function balanceOf(address _owner) public view returns (uint256 balance){
        balance = _balances[_owner];
    }

    function transfer(address _to, uint256 _value) public returns (bool success){
        require(_balances[msg.sender] >= _value);
        _balances[msg.sender] -= _value;
        _balances[_to] += _value;
        Transfer(msg.sender, _to, _value);
        success = true;
    }

    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success){
        require(_balances[_from] >= _value);
        require(_allowances[_from][msg.sender] >= _value);
        _balances[_from] -= _value;
        _allowances[_from][msg.sender] -= _value;
        _balances[_to] += _value;
        Transfer(_from, _to, _value);
        success = true;
    }

    function approve(address _spender, uint256 _value) public returns (bool success){
        _allowances[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        success = true;
    }

    function allowance(address _owner, address _spender) public view returns (uint256 remaining){
        remaining = _allowances[_owner][_spender];
    }
}

代碼說明

• 代幣名稱為 “MyToken”
• 代幣標(biāo)識符為 “MYT”
• 小數(shù)位數(shù)為3
• 代幣初始總供給為 10 * 1000 = 10000
• 賬戶余額映射
• 賬戶配額映射榴啸，這里是兩層映射，例如賬戶A授信給賬戶B額度為value的配額
• 代幣轉(zhuǎn)移事件倦蚪，事件內(nèi)容為：將賬戶_from的代幣轉(zhuǎn)移到賬戶_to中弟头，金額為_value彻坛。indexed關(guān)鍵字是對事件的字段做索引珍逸，具體用途后面會說僧免。
• 配額許可事件俭缓，事件內(nèi)容為：擁有者_owner許可給_spender配額克伊，額度為_value
• 構(gòu)造方法，將代幣的初始總供給都分配給合約的部署賬戶华坦。合約的構(gòu)造方法只在合約部署時執(zhí)行一次愿吹。
• 公共方法（合約外部可訪問），返回合約名稱惜姐。view關(guān)鍵子表示函數(shù)只查詢狀態(tài)變量犁跪，而不寫入。函數(shù)的view關(guān)鍵字與constant等價歹袁。
• 公共方法坷衍，返回合約標(biāo)識符。
• 公共方法条舔，返回合約小數(shù)位枫耳。
• 公共方法，返回合約總供給額孟抗。
• 公共方法迁杨，查詢賬戶_owner的賬戶余額钻心。
• 公共方法，將合約調(diào)用者的代幣轉(zhuǎn)移到賬戶_to中铅协，金額為_value捷沸，轉(zhuǎn)移成功返回true。
• 驗證合約調(diào)用者（msg.sender）賬戶余額是否大于等于_value
• 合約調(diào)用者賬戶余額減去_value
• 接收賬戶_to賬戶余額額加上_value
• 發(fā)送Transfer()事件
• 返回轉(zhuǎn)移成功（true）
• 公共方法狐史，將賬戶_from的代幣轉(zhuǎn)移到賬戶_to中痒给，金額為_value，轉(zhuǎn)移成功返回true
• 驗證賬戶_from的代幣余額是否大于等于_value
• 驗證賬戶_from給合約調(diào)用者msg.sender的配額是否大于等于_value
• 賬戶_from的賬戶余額減去_value
• 賬戶_from給合約調(diào)用者（msg.sender）的配額減去_value
• 接收賬戶_to賬戶余額額加上_value
• 發(fā)送Transfer()事件
• 返回轉(zhuǎn)移成功true
• 公共方法预皇，合約調(diào)用者批準(zhǔn)給賬戶_spender配額侈玄，額度為_value，批準(zhǔn)成功返回true
• 合約調(diào)用者msg.sender對賬戶_spender的配額等于_value
• 發(fā)送Approval()事件
• 返回批準(zhǔn)成功true
• 公共方法吟温，查詢賬戶_owner批準(zhǔn)給賬戶_spender的配額
• 返回賬戶_owner批準(zhǔn)給賬戶_spender的配額

參考文檔
https://github.com/ConsenSys/Tokens/blob/master/contracts/eip20/EIP20Interface.sol
https://github.com/ConsenSys/Tokens/blob/master/contracts/eip20/EIP20.sol