#include "iostream"

using namespace std;


union A
{
    struct B
    {
        unsigned int x1:2;
        unsigned int x2:3;
        unsigned int x3:3;
    };
   
    char a;
    B b;
};
   
   
int main(int argc, char *argv[])
{
A a;
a.a=100;

cout<<a.b.x1<<endl<<a.b.x2<<endl<<a.b.x3<<endl;

system("pause");
return 0;
}

//结果   0 1 3

首先要明白union联合的意思
A的长度为8(2+3+3)
所以{x3,x2,x1} = 100(d) = 011 001 00(b)
很明显:
x1 = 0;
x2 = 1;
x3 = 3;

7楼兜兜的正解