区块链学堂（29）：Modifiers

待我称王封你为后i 2022-06-02 05:38 210阅读 0赞

> Modifiers can be used to easily change the behaviour of functions, for example to automatically check a condition prior to executing the function. They are inheritable properties of contracts and may be overridden by derived contracts. 引用自[here][]

### 一个使用modifier的例子 ###

pragma solidity 0.4.10;
    contract demo {
        uint public deadline;
        function demo() {
            deadline = now;
            deadline += 10;
        }
    
        modifier afterDeadline {
            if (now <= deadline) throw;
            _;
        }
    
        function f() afterDeadline returns (string s) {
            return "afterDeadline";
        }
    
        function g() afterDeadline returns (uint n) {
            n = 100;
        }
    }

*  从上面的例子中可以看到，我们先在构造函数中，给deadline赋予一个值。在当前时间+10s
 *  然后modifier afterDeadline, 必须要当前时间过了10s，才不抛出异常
 *  然后我们可以在很多方法中使用modifier afterDeadline(). 从而使得代码非常简洁和美观。

### 另一个例子，给上一节中的代币合约中添加Modifier ###

*  Step 1: 首先定义一个modifier
    
        modifier onlyMinter {
            if (msg.sender != minter) throw;
            _;
          }
 *  Step 2: 新增一个新的 function mint2()

function changeMinter(address _to) onlyMinter {
          minter = _to;
        }

对比一下之前的function mint()

function mint(address receiver, uint amount) {
            if (msg.sender != minter) throw;
            balances[receiver] += amount;
        }

很明显可以看到，通过modifier，在执行主函数之前，先进行一个条件检查。通过modifier,可以使代码变得很美。

*  Step 3: 添加个function changeMinter()
    
        function changeMinter(address _to) onlyMinter {
            minter = _to;
          }

##### 上一节代码加上Step 1-3后的完整代码如下： #####

pragma solidity 0.4.7;
    
    contract Coin {
        address public minter;
        mapping (address => uint) public balances;
    
        event Sent(address from, address to, uint amount);
    
        function Coin() {
            minter = msg.sender;
        }
    
        function mint(address receiver, uint amount) {
            if (msg.sender != minter) throw;
            balances[receiver] += amount;
        }
    
        function send(address receiver, uint amount) {
            if (balances[msg.sender] < amount) return;
            balances[msg.sender] -= amount;
            balances[receiver] += amount;
            Sent(msg.sender, receiver, amount);
        }
    
        //对挖矿使用onlyOwner
        modifier onlyMinter {
          if (msg.sender != minter) throw;
          _;
        }
    
        function mint2(address receiver, uint amount) onlyMinter {
          balances[receiver] += amount;
        }
    
        function changeMinter(address _to) onlyMinter {
          minter = _to;
        }
    }

--------------------

原文：http://www.ethchinese.com/?p=1122

QQ群：559649971 （区块链学堂粉丝群)  
个人微信：steven\_k\_colin

![stevenkcolin.jpg][]

获取最新区块链咨询，请关注《以太中文网》微信公众号：以太中文网

![ethchinese.jpg][]

[here]: https://solidity.readthedocs.io/en/develop/contracts.html
[stevenkcolin.jpg]: http://www.ethchinese.com/wp-content/uploads/2017/02/stevenkcolin.jpg
[ethchinese.jpg]: http://www.ethchinese.com/wp-content/uploads/2017/02/ethchinese.jpg