C++结构体字节对齐

#include <iostream>    //输入输出流
using namespace std;
#include <vector>      //容器
struct MyStruct
{
   unsigned char start1;
   unsigned char start2;
   unsigned char test1[7];
};
struct header
{
   unsigned char by; //1字节
   unsigned int dw; //4字节
   int flag; //4字节
};
typedef unsigned char UCHAR;    //定义无符号字符变量的别名
typedef unsigned int UINT;      //定义无符号整型变量的别名
const int PREAMBLE_LENGTH = 7;   //定义整型常量，前导码占7个字节
const int START_LENGTH = 1;      //帧前定界符占1个字节
const int DESADD_LENGTH = 6;     //目的地址
const int SOURCEADD_LENGTH = 6;  //源地址
const int TYPE_LENGTH = 2;       //类型字段
const int FCS_LENGTH = 4;        //帧校验地址
class MyFrame1
{
public:
   UCHAR preamble[PREAMBLE_LENGTH];     //前导码
   UCHAR start;                         //帧前定界符,标明下一个字节为目的MAC字段
   UCHAR dest_add[DESADD_LENGTH];       //指明帧的目的地址
   UCHAR source_add[SOURCEADD_LENGTH];  //指明帧的源地址
   UCHAR type[TYPE_LENGTH];             //帧中数据的协议类型
   vector <UCHAR> data;                  //vector <UCHAR>占16字节    定义数据的容器存储数据字段
   UCHAR fcs[FCS_LENGTH];               //帧校验序列
};
int main()
{
   MyStruct struct1;
   cout << sizeof(struct1)<<endl;
   header str1;
   cout << sizeof(str1) << endl;
   MyFrame1 fram1;
   cout << sizeof(fram1) << endl;
    return 0;
}

#include <iostream>    //输入输出流
using namespace std;
#include <vector>      //容器
#pragma pack(1)         #按1个字节对齐，这样相当于就没有内存对齐