一天一个CRT函数 _swap

这是Buffer Manipulation中最后一个函数了，和我们STL里所用的swap函数极为相似。看下函数声明：

void _swab(   char *src,   char *dest,   int n );

如果n为奇数，则把src中n字节的数据与dest中n字节的数据相互交换。如果n为偶数，则把src中n-1字节的数据与dest的n-1字节的数据相互交换。

MSDN上还说“_swab is typically used to prepare binary data for transfer to a machine that uses a different byte order.”

看，在网络传输中还是有一定地位的。

1.实现

   1:  

   2:     /***

   3:     *

   4:     *Purpose:

   5:     *       This routine copys a block of words and swaps the odd and even

   6:     *       bytes.  nbytes must be > 0, otherwise nothing is copied.  If

   7:     *       nbytes is odd, then only (nbytes-1) bytes are copied.

   8:     *

   9:     *Entry:

  10:     *       srcptr = pointer to the source block

  11:     *       dstptr = pointer to the destination block

  12:     *       nbytes = number of bytes to swap

  13:     *

  14:     *Returns:

  15:     *       None.

  16:     *

  17:     *Exceptions:

  18:     *

  19:     ***/

  20:     template<typename T>

  21:     void tSwab(T *pSrc, T *pDest, int nBytes)

  22:     {

  23:         tChar b1 = _T('/0');

  24:         tChar b2 = _T('/0');

  25:  

  26:         assert(pSrc != NULL);

  27:         assert(pDest != NULL);

  28:         assert(nBytes >= 0);

  29:  

  30:         while( nBytes > 1 )

  31:         {

  32:             b1 = *pSrc++;

  33:             b2 = *pSrc++;

  34:             *pDest++ = b2;

  35:             *pDest++ = b1;

  36:  

  37:             nBytes -= 2;

  38:         }

  39:     }

看来还是用到了中间变量，而且还是俩！

2.测试

   1: volatile tChar from[] = _T("BADCFEHGJILKNMPORQTSVUXWZYBADCFEHGJILKNMPORQTSVUXWZY");

   2: volatile tChar to[] =   _T("....................................................");

   3:  

   4:  

   5: volatile const DWORD dwCount = 10000000;

   6: int nSize = sizeof( from ) / sizeof(from[0]);

   7:  

   8: DWORD dwLast = 0; 

   9: {

  10:     CCYPerformance timer(dwLast);

  11:     for(DWORD i = 0; i < dwCount; ++i)

  12:     {

  13:         CY_CRT::tSwab(from, to, nSize);

  14:     }

  15: }

  16: cout << dwLast << endl;

  17:  

  18: dwLast = 0; 

  19: {

  20:     CCYPerformance timer(dwLast);

  21:     for(DWORD i = 0; i < dwCount; ++i)

  22:     {

  23:         ::_swab((char *)from, (char *)to, nSize);

  24:     }

  25: }

  26: cout << dwLast << endl;

 

结果：