# 【OpenZeppelin】

OpenZeppelin 是一個智能合約庫，其中包含了經過社區審查的 ERC 代幣標準、安全協議以及其他輔助工具，使用 OpenZeppelin 可以極大的提高 Solidity 合約的開發效率並保證合約的安全性。最常用的合約庫如下：

* Ownable：
  * OpenZeppelin 的 Ownable 合約提供的 onlyOwner 模式是用來保護特定合約方法的訪問權限的基礎但非常有效的模式。
* SafeMath：
  * 這個合約會進行所有必要的檢查，避免你的程式碼因算術運算而出現漏洞，或者因為溢位等因素而出現預期外的計算結果。
* SafeCast：
  * 當我們在做型別轉換的時候有可能會出現精度遺漏或者溢位等問題，SafeCast 可以幫我們完成這些轉換而無需擔心溢出問題。
* ERC20/ERC721 等代幣模式

#### Libraries & Using ...

[OpenZeppelin](https://github.com/OpenZeppelin/openzeppelin-contracts)

```solidity
pragma solidity ^0.5.13;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/math/SafeMath.sol";

contract LibrariesExample {
    
    using SafeMath for uint;

    mapping(address => uint) public tokenBalance;

    constructor() public {
        tokenBalance[msg.sender] = tokenBalance[msg.sender].add(1);
        // tokenBalance[msg.sender] = 1;
    }

    function sendToken(address _to, uint _amount) public returns(bool) {
        tokenBalance[msg.sender] = tokenBalance[msg.sender].sub(_amount);
        tokenBalance[_to] =  tokenBalance[_to].add(_amount); 
        // tokenBalance[msg.sender] -= _amount;
        // tokenBalance[_to] += _amount;

        return true;
    }
}
```

< 0.8 or >= 0.8 has differnet overfolw and underflow problem

```solidity
pragma solidity >=0.4.16 <0.7.0;

library Search {
    
    function indexOf(uint[] storage self, uint value)
    public
    view
    returns (uint){
        for (uint i = 0; i < self.length; i++)
            if (self[i] == value) return i;
            return uint(-1);
        }
}

contract NotUsingForExample {
    
    uint[] data;
    
    function append(uint value) public {
        data.push(value);    
    }
    
    function replace(uint _old, uint _new) public {
    // This performs the library function call
        uint index = Search.indexOf(data,_old);
        if (index == uint(-1))
            data.push(_new);
        else
            data[index] = _new;
    }
}

contract UsingForExample {
    
    using Search for uint[];
    uint[] data;
    
    function append(uint value) public {
        data.push(value);
    }
    
    function replace(uint _old, uint _new) public {
        // This performs the library function call
        uint index = data.indexOf(_old);
        if (index == uint(-1))
            data.push(_new);
        else
            data[index] = _new;
    }
}
```

使用OpenZeppelin需要特別注意GitHub上的分支，尤其是版本不同在引入到智能合約時會導致編譯錯誤或各種問題。舊版的OpenZeppelin使用上也需要格外小心，因為更新版本有可能是為了修補過往的漏洞或改善品質。


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