const int * a // a是一个指针，指向一个const int类型的内存， a本身可以修改指向别的变量，但是a所指向的内存中的数据不能修改。
int const * a // 和第一种相同
int * const a // const修饰的是a， a是一个int *型的变量。也就是说a本身不能修改，即指向确定的内存，但所指向的内存中的数据可以修改。
const int * const a // a以及a所指向的内存中的值都不能被修改。

#include <iostream>
#include <cstddef>

using namespace std;

struct a{
        char a;
        int b;
        short c;
        double d;
};

struct b{
        char a;
        short b;
        int c;
        float d;
};

int main()
{
        cout<<"sizeof(struct a) : "<<sizeof(struct a)<<" sizeof(struct b) : "<<sizeof(struct b)<<endl;
        cout<<"sizeof(short) : "<<sizeof(short)<<endl;
        cout<<"sizeof(int) : "<<sizeof(int)<<endl;
        cout<<"sizeof(long int) : "<<sizeof(long int)<<endl;
        cout<<"sizeof(long long int) : "<<sizeof(long long int)<<endl;
        cout<<"sizeof(flaot) : "<<sizeof(float)<<endl;
        cout<<"sizeof(double) : "<<sizeof(double)<<endl;


        printf("offset of b in struct a : %lun", offsetof(struct a, b));
        printf("offset of d in struct a : %lun", offsetof(struct a, d));
        printf("offset of b in struct b : %lun", offsetof(struct b, b));

        return 0;
}
程序在64位机器上的输出结果为
sizeof(struct a) : 24 sizeof(struct b) : 12
sizeof(short) : 2
sizeof(int) : 4
sizeof(long int) : 8
sizeof(long long int) : 8
sizeof(flaot) : 4
sizeof(double) : 8
offset of b in struct a : 4
offset of d in struct a : 16
offset of b in struct b : 2

#include <iostream>
#include <cstdio>

using namespace std;

int main()
{
	int array[2019]={0};
	array[19] = 2019;

	int offset = (unsigned long)((short *)array+2019) - (unsigned long)array + *(unsigned char *)(array+19);

	cout<<offset<<endl;

	printf("%pn",  2019);

	printf("%pn", array);
	cout<<(unsigned long)(array)<<endl;
	cout<<(unsigned long)((short *)array+2019)<<endl;
	cout<<(unsigned long)array+*(unsigned char *)(array+19)<<endl;
	cout<<(int)*(unsigned char *)(array+19)<<endl;

    return 0;
}
输出结果如下
4265
0x7e3						// 2019的十六进制，小端存储模式，所以在内存中应为 e3 07, 故*(unsigned char *)(array+19) = 0xe3 = 227
0x7ffeec55a800  // 64位机器输出的地址是64位的
140732863457280 // array的起始地址
140732863461318 // short占用两个字节 (unsigned long)((short *)array+2019)相当于array向后偏移了2*2019=4038个字节，因此地址值为array+4038
140732863457507 // 
227

#include <iostream>
#include <string>

using namespace std;

class A{
public:
        A(){}
        ~A(){}

        void p(const string & s)
        {
                cout<<s<<endl;
        }
};

int main()
{
        A * p = NULL;
        p->p("hello worldn");
}