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在數(shù)學運算過程中假如超過了長度則值會變成該類型的最小值，如果小于了該長度則變成最大值

• 數(shù)據(jù)上溢

uint8 numA = 255;
numA++;

?uint8的定義域為[0,255]，現(xiàn)在numA已經(jīng)到頂了，numA++會使num變成0(由于256已經(jīng)超過定義域，它會越過256，變成0)，即數(shù)據(jù)發(fā)生上溢(越過上邊界，叫上溢)。255 --> 256 -->0 上溢。

• 數(shù)據(jù)下溢

uint8 numB = 0;
numB--;

numB本身是低水位線，現(xiàn)在numB-- 會使num變成255(由于-1已經(jīng)超過定義域，所以它會越過-1，變成255)，即數(shù)據(jù)發(fā)生下溢(越過下邊界，叫下溢)。0–> -1 --> 255 下溢。??

?可以通過引用 OpenZeppelin的 SafeMath v2.5.x 庫，或者自定義一個SafeMath合約，來避免該問題。
????庫是 Solidity 中一種特殊的合約，它給原始數(shù)據(jù)類型增加了一些方法: add()， sub()， mul()， 以及 div()。
????先引用或者import SafeMath庫，然后聲明 using SafeMath for uint256 ，再通過變量名來調(diào)用這些方法。
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方法1：

導入庫import "SafeMath" ?

給變量使用庫 using SafeMath for 變量類型

傳遞參數(shù)給庫中add函數(shù)

uint e=255;

e=參數(shù)1.add(參數(shù)2)? ? ? ? ? ?底層是?參數(shù)1傳給a，參數(shù)2傳給b?

方法2：

也可以自己創(chuàng)建一個庫，用library聲明，而不是contract

library SafeMath {

func add(uint8 a,uint b) internal pure returns(uint 8){ 檢驗加法

? ? ? ?uint8 = a+b;

? ? ? ?assert(c>=a);

? ? ? ?assert()函數(shù)可以用來判斷參數(shù)是否成立，若不成立則彈出一個錯誤

? ? ? ?return c;}}

試例

import "./safemath.sol";          //1)引用庫
using SafeMath for uint256;       //2)聲明指定的類型uint256 a = 5;
uint256 b = a.add(3); // 5 + 3 = 8  //3)用變量名來調(diào)用方法
uint256 c = a.mul(2); // 5 * 2 = 10

庫合約源碼

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

簡單變量

uint numA;
numA++;

優(yōu)化后?

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

map

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

優(yōu)化后?

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

結(jié)構(gòu)體?

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

優(yōu)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.自減修改

簡單變量

uint8 numB;
numB--;

優(yōu)化后?

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