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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 0uint256 c = a / b;// assert(a == b * c + a % b); // There is no case in which this doesn't holdreturn 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 0uint32 c = a / b;// assert(a == b * c + a % b); // There is no case in which this doesn't holdreturn 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 0uint16 c = a / b;// assert(a == b * c + a % b); // There is no case in which this doesn't holdreturn 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 0uint8 c = a / b;// assert(a == b * c + a % b); // There is no case in which this doesn't holdreturn 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);