【记录】C++中的位域

代码如下：

// 临界区.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <windows.h>

int _tmain(int argc, _TCHAR* argv[])
{
	struct bs
	{
		unsigned a : 1;
		unsigned b : 3;
		unsigned c : 32;
	}bit,*pbit;
	bit.a = 1;
	bit.b = 7;
	bit.c = 15;
	//printf("%d,%d,%d", bit.a, bit.b, bit.c);
	pbit = &bit;
	pbit->a = 0;
	pbit->b &= 3;
	pbit->c |= 1;
	printf("%d,%d,%d", pbit->a, pbit->b, pbit->c);
	getchar();
	return 0;
}

　　位域主要的位数的定义，最大可到255，定义到32位。

看看反汇编就都明白了。

00053CA0 55                   push        ebp  
00053CA1 8B EC                mov         ebp,esp  
00053CA3 81 EC E0 00 00 00    sub         esp,0E0h  
00053CA9 53                   push        ebx  
00053CAA 56                   push        esi  
00053CAB 57                   push        edi  
00053CAC 8D BD 20 FF FF FF    lea         edi,[ebp-0E0h]  
00053CB2 B9 38 00 00 00       mov         ecx,38h  
00053CB7 B8 CC CC CC CC       mov         eax,0CCCCCCCCh  
00053CBC F3 AB                rep stos    dword ptr es:[edi]  
00053CBE A1 24 80 05 00       mov         eax,dword ptr ds:[00058024h]  
00053CC3 33 C5                xor         eax,ebp  
00053CC5 89 45 FC             mov         dword ptr [ebp-4],eax  
     9: 	struct bs
    10: 	{
    11: 		unsigned a : 1;
    12: 		unsigned b : 3;
    13: 		unsigned c : 32;
    14: 	}bit,*pbit;
    15: 	bit.a = 1;
00053CC8 8B 45 F0             mov         eax,dword ptr [bit]  
00053CCB 83 C8 01             or          eax,1  
00053CCE 89 45 F0             mov         dword ptr [bit],eax  
    16: 	bit.b = 7;
00053CD1 8B 45 F0             mov         eax,dword ptr [bit]  
00053CD4 83 C8 0E             or          eax,0Eh  
00053CD7 89 45 F0             mov         dword ptr [bit],eax  
    17: 	bit.c = 15;
00053CDA 8B 45 F4             mov         eax,dword ptr [ebp-0Ch]  
00053CDD 83 E0 00             and         eax,0  
00053CE0 83 C8 0F             or          eax,0Fh  
00053CE3 89 45 F4             mov         dword ptr [ebp-0Ch],eax  
    18: 	//printf("%d,%d,%d", bit.a, bit.b, bit.c);
    19: 	pbit = &bit;
00053CE6 8D 45 F0             lea         eax,[bit]  
00053CE9 89 45 E4             mov         dword ptr [pbit],eax  
    20: 	pbit->a = 0;
00053CEC 8B 45 E4             mov         eax,dword ptr [pbit]  
00053CEF 8B 08                mov         ecx,dword ptr [eax]  
00053CF1 83 E1 FE             and         ecx,0FFFFFFFEh  
00053CF4 8B 55 E4             mov         edx,dword ptr [pbit]  
00053CF7 89 0A                mov         dword ptr [edx],ecx  
    21: 	pbit->b &= 3;
00053CF9 8B 45 E4             mov         eax,dword ptr [pbit]  
00053CFC 8B 08                mov         ecx,dword ptr [eax]  
00053CFE D1 E9                shr         ecx,1  
00053D00 83 E1 07             and         ecx,7  
00053D03 83 E1 03             and         ecx,3  
00053D06 83 E1 07             and         ecx,7  
00053D09 D1 E1                shl         ecx,1  
00053D0B 8B 55 E4             mov         edx,dword ptr [pbit]  
00053D0E 8B 02                mov         eax,dword ptr [edx]  
00053D10 83 E0 F1             and         eax,0FFFFFFF1h  
00053D13 0B C1                or          eax,ecx  
00053D15 8B 4D E4             mov         ecx,dword ptr [pbit]  
00053D18 89 01                mov         dword ptr [ecx],eax  
    22: 	pbit->c |= 1;
00053D1A 8B 45 E4             mov         eax,dword ptr [pbit]  
00053D1D 8B 48 04             mov         ecx,dword ptr [eax+4]  
00053D20 83 C9 01             or          ecx,1  
00053D23 8B 55 E4             mov         edx,dword ptr [pbit]  
00053D26 8B 42 04             mov         eax,dword ptr [edx+4]  
00053D29 83 E0 00             and         eax,0  
00053D2C 0B C1                or          eax,ecx  
00053D2E 8B 4D E4             mov         ecx,dword ptr [pbit]  
00053D31 89 41 04             mov         dword ptr [ecx+4],eax  
    23: 	printf("%d,%d,%d", pbit->a, pbit->b, pbit->c);
00053D34 8B 45 E4             mov         eax,dword ptr [pbit]  
00053D37 8B 48 04             mov         ecx,dword ptr [eax+4]  
00053D3A 8B F4                mov         esi,esp  
00053D3C 51                   push        ecx  
00053D3D 8B 55 E4             mov         edx,dword ptr [pbit]  
00053D40 8B 02                mov         eax,dword ptr [edx]  
00053D42 D1 E8                shr         eax,1  
00053D44 83 E0 07             and         eax,7  
00053D47 50                   push        eax  
00053D48 8B 4D E4             mov         ecx,dword ptr [pbit]  
00053D4B 8B 11                mov         edx,dword ptr [ecx]  
00053D4D 83 E2 01             and         edx,1  
00053D50 52                   push        edx  
00053D51 68 C0 58 05 00       push        558C0h  
00053D56 FF 15 24 91 05 00    call        dword ptr ds:[59124h]  
00053D5C 83 C4 10             add         esp,10h  
00053D5F 3B F4                cmp         esi,esp  
00053D61 E8 DA D3 FF FF       call        __RTC_CheckEsp (051140h)  

　　在反汇编中我们可以看到当申明struct结构体的时候里面的所有变量操作并不占用空间。

当对结构体成员等变量操作的时候才进行占用空间。

同时所有定义的十进制在结构体中都会转换成二进制进行位域比较。