_BitScanForward, _BitScanForward64

_BitScanForward(64)的使用得include头文件intrin.h
unsigned char _BitScanForward(
   unsigned long * Index,
   unsigned long Mask
);
unsigned char_BitScanForward64(
   unsigned long * Index,
   unsigned __int64 Mask
);

_BitScanForward(64)函数的作用是以2进制的形式从Mask的最低有效位（least significant bit）向最高有效位（most significant bit ）搜索，直到找到1.

eg.  

　　unsigned long index0, index1, index2;

　　unsigned long mask0 =   1B;

　　unsinged long mask1 =   100B;

　　unsigned long mask2 = 1100B;

　　_BitScanForward(&index0, mask0);

　　_BitScanForward(&index1, mask1);

　　_BitScanForward(&index2, mask2);

得到的结果是:

　　index0 = 0; //因为1出现在第一位

　　index1=index2 = 2；//100B和1100B的第一个1都是出现在第三位

note：在编程中计数都是从0开始，所以出现在第3位的1得到的结果是2.

#include <iostream>
#include <intrin.h>
using namespace std;

#pragma intrinsic(_BitScanForward)

int main()
{
    unsigned long mask = 0x1000;
    unsigned long index;
    unsigned char isNonzero;

    cout << "Enter a positive integer as the mask: " << flush;
    cin >> mask;
    isNonzero = _BitScanForward(&index, mask);
    if (isNonzero)
    {
        cout << "Mask: " << mask << " Index: " << index << endl;
    }
    else
    {
        cout << "No set bits found.  Mask is zero." << endl;
    }
    return 0;
}