Solidity拓展:数学运算过程中数据长度溢出的问题

在数学运算过程中假如超过了长度则值会变成该类型的最小值，如果小于了该长度则变成最大值

• 数据上溢

uint8 numA = 255;
numA++;

 uint8的定义域为[0,255]，现在numA已经到顶了，numA++会使num变成0(由于256已经超过定义域，它会越过256，变成0)，即数据发生上溢(越过上边界，叫上溢)。255 --> 256 -->0 上溢。

• 数据下溢

uint8 numB = 0;
numB--;

numB本身是低水位线，现在numB-- 会使num变成255(由于-1已经超过定义域，所以它会越过-1，变成255)，即数据发生下溢(越过下边界，叫下溢)。0–> -1 --> 255 下溢。  

 可以通过引用 OpenZeppelin的 SafeMath v2.5.x 库，或者自定义一个SafeMath合约，来避免该问题。
    库是 Solidity 中一种特殊的合约，它给原始数据类型增加了一些方法: add()， sub()， mul()， 以及 div()。
    先引用或者import SafeMath库，然后声明 using SafeMath for uint256 ，再通过变量名来调用这些方法。
 

方法1：

导入库import "SafeMath"  

给变量使用库 using SafeMath for 变量类型

传递参数给库中add函数

uint e=255;

e=参数1.add(参数2)           底层是 参数1传给a，参数2传给b 

方法2：

也可以自己创建一个库，用library声明，而不是contract

library SafeMath {

func add(uint8 a,uint b) internal pure returns(uint 8){ 检验加法

       uint8 = a+b;

       assert(c>=a);

       assert()函数可以用来判断参数是否成立，若不成立则弹出一个错误

       return c;}}

试例

import "./safemath.sol";          //1)引用库
using SafeMath for uint256;       //2)声明指定的类型
 
uint256 a = 5;
uint256 b = a.add(3); // 5 + 3 = 8  //3)用变量名来调用方法
uint256 c = a.mul(2); // 5 * 2 = 10

库合约源码

pragma solidity ^0.4.18;
 
/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
 
  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }
 
  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }
 
  /**
  * @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }
 
  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}
 
/**
 * @title SafeMath32
 * @dev SafeMath library implemented for uint32
 */
library SafeMath32 {
 
  function mul(uint32 a, uint32 b) internal pure returns (uint32) {
    if (a == 0) {
      return 0;
    }
    uint32 c = a * b;
    assert(c / a == b);
    return c;
  }
 
  function div(uint32 a, uint32 b) internal pure returns (uint32) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint32 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }
 
  function sub(uint32 a, uint32 b) internal pure returns (uint32) {
    assert(b <= a);
    return a - b;
  }
 
  function add(uint32 a, uint32 b) internal pure returns (uint32) {
    uint32 c = a + b;
    assert(c >= a);
    return c;
  }
}
 
/**
 * @title SafeMath16
 * @dev SafeMath library implemented for uint16
 */
library SafeMath16 {
 
  function mul(uint16 a, uint16 b) internal pure returns (uint16) {
    if (a == 0) {
      return 0;
    }
    uint16 c = a * b;
    assert(c / a == b);
    return c;
  }
 
  function div(uint16 a, uint16 b) internal pure returns (uint16) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint16 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }
 
  function sub(uint16 a, uint16 b) internal pure returns (uint16) {
    assert(b <= a);
    return a - b;
  }
 
  function add(uint16 a, uint16 b) internal pure returns (uint16) {
    uint16 c = a + b;
    assert(c >= a);
    return c;
  }
}
 
library SafeMath8 {
 
  function mul(uint8 a, uint8 b) internal pure returns (uint8) {
    if (a == 0) {
      return 0;
    }
    uint8 c = a * b;
    assert(c / a == b);
    return c;
  }
 
  function div(uint8 a, uint8 b) internal pure returns (uint8) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint8 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }
 
  function sub(uint8 a, uint8 b) internal pure returns (uint8) {
    assert(b <= a);
    return a - b;
  }
 
  function add(uint8 a, uint8 b) internal pure returns (uint8) {
    uint8 c = a + b;
    assert(c >= a);
    return c;
  }
}
 

1.自增修改

简单变量

uint numA;
numA++;

优化后 

import "./safemath.sol";
using SafeMath for uint256;
 
uint numA;
//numA++;
numA = numA.add(1);

map

mapping(address => uint) ownerAppleCount;
ownerAppleCount[msg.sender]++;

优化后 

import "./safemath.sol";
using SafeMath for uint256;
 
mapping(address => uint) ownerAppleCount;
//ownerAppleCount[msg.sender]++;
ownerAppleCount[msg.sender] = ownerAppleCount[msg.sender].add(1);

结构体 

struct Apple {
    uint32 id;	
	uint   weight;
	string color;
}
Apple zhaoApple = Apple(100,150,"red");
zhaoApple.weight++;

优化后 

import "./safemath.sol";
using SafeMath for uint256;
using SafeMath32 for uint32;
 
struct Apple {
    uint32 id;	
	uint   weight;
	string color;
}
Apple zhaoApple = Apple(100,150,"red");
//zhaoApple.weight++;
zhaoApple.weight = zhaoApple.weight.add(1);

2.自减修改

简单变量

uint8 numB;
numB--;

优化后 

import "./safemath.sol";
using SafeMath8 for uint8;
 
uint8 numB;
//numB--;
numB = numB.sub(1);