union的一些问题

[kuailexiaoziwqx]

/*为什么
A={“a”}  A.l=97
A={“aa”}  A.l=24929  怎么得来的呢
A={“ab”}  A.l=25185   与上一个 数值差256 

*/
#include <stdio.h>
#include<iostream>
using namespace std;
union a
{
	char str[4];
	 int l;
}A={"a"};
int main()
{	cout<<A.l;
	system("pause");
    return 0; 
}

[Sniper_Pan]

union占内存为最大变量所占空间，本段代码中X86情况下通常为4字节。 当A={"a"}时，内部存储a的ASCII值，0x61，十进制显示后为97 当A={"aa"}时，内部存储值为0x6161，十进制显示后为24929 当A={“ab”}时，内部存储值为0x6261（小端字节序），十进制显示后为25185

[窗外蓝天]

A={"a"} 0-----------32 61 00 00 00，l=0x00000061 A={"aa"} 0-----------32 61 61 00 00，l=0x00006161 A={"ab"} 0-----------32 61 62 00 00，l=0x00006261

[xskxzr]

见http://en.cppreference.com/w/cpp/language/union（标红是我加的重点）：

引用

The union is only as big as necessary to hold its largest data member. The other data members are allocated in the same bytes as part of that largest member. The details of that allocation are implementation-defined, and it's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union.

楼上一群人一本正经地研究非标准扩展，真的好吗？

[赵4老师]

仅供参考：

#include <stdio.h>
#include <stdlib.h>
char buf[17];
union U {
  unsigned short int aa;
  struct {
    unsigned int bb:7;//(bit 0-6)
    unsigned int cc:6;//(bit 7-12)
    unsigned int dd:3;//(bit 13-15)
  };
} u;
void main() {
                //bbbbbbbbbbbbbbbb
                //iiiiiiiiiiiiiiii
                //tttttttttttttttt
                //111111
                //5432109876543210
                //::::::::::::::::
    u.aa=0xE07F;//1110000001111111
    printf("bb==%d,cc==%d,dd==%d\n",u.bb,u.cc,u.dd);
    u.bb=0x41;
    u.cc=0x21;//dddccccccbbbbbbb
    u.dd=5;   //1011000011000001
    printf("aa==0x%04X==%016s(2)\n",u.aa,itoa(u.aa,buf,2));
}
//bb==127,cc==0,dd==7
//aa==0xB0C1==1011000011000001(2)

[赵4老师]

再供参考：

void HexDump(char *buf,int len,int addr) {
    int i,j,k;
    char binstr[80];

    for (i=0;i<len;i++) {
        if (0==(i%16)) {
            sprintf(binstr,"%08x -",i+addr);
            sprintf(binstr,"%s %02x",binstr,(unsigned char)buf[i]);
        } else if (15==(i%16)) {
            sprintf(binstr,"%s %02x",binstr,(unsigned char)buf[i]);
            sprintf(binstr,"%s  ",binstr);
            for (j=i-15;j<=i;j++) {
                sprintf(binstr,"%s%c",binstr,('!'<buf[j]&&buf[j]<='~')?buf[j]:'.');
            }
            printf("%s\n",binstr);
        } else {
            sprintf(binstr,"%s %02x",binstr,(unsigned char)buf[i]);
        }
    }
    if (0!=(i%16)) {
        k=16-(i%16);
        for (j=0;j<k;j++) {
            sprintf(binstr,"%s   ",binstr);
        }
        sprintf(binstr,"%s  ",binstr);
        k=16-k;
        for (j=i-k;j<i;j++) {
            sprintf(binstr,"%s%c",binstr,('!'<buf[j]&&buf[j]<='~')?buf[j]:'.');
        }
        printf("%s\n",binstr);
    }
}