谨慎使用A2W等字符转换宏

在一个函数的循环体中使用A2W等字符转换宏可能引起栈溢出。

#include <atlconv.h>
void fn()
{
    while(true)
    {
        {
            USES_CONVERSION;
            DoSomething(A2W("SomeString"));
        }
    }
}

让我们来分析以上的转换宏

#define A2W(lpa) (
   ((_lpa = lpa) == NULL) ? NULL : (
      _convert = (lstrlenA(_lpa)+1),
      ATLA2WHELPER((LPWSTR) alloca(_convert*2), _lpa, _convert)))

#define ATLA2WHELPER AtlA2WHelper

inline LPWSTR WINAPI AtlA2WHelper(LPWSTR lpw, LPCSTR lpa, int nChars, UINT acp)
{
   ATLASSERT(lpa != NULL);
   ATLASSERT(lpw != NULL);
   // verify that no illegal character present
   // since lpw was allocated based on the size of lpa
   // don't worry about the number of chars
   lpw[0] = '';
   MultiByteToWideChar(acp, 0, lpa, -1, lpw, nChars);
   return lpw;
}

关键的地方在 alloca  内存分配内存上。
#define alloca  _alloca

_alloca
Allocates memory on the stack.

Remarks
_alloca allocates size bytes from the program stack. The allocated space is automatically freed when the calling function

exits. Therefore, do not pass the pointer value returned by _alloca as an argument to free.

问题就在这里，分配的内存是在函数的栈中分配的。而VC编译器默认的栈内存空间是2M。当在一个函数中循环调用它时就会不断的分配栈中的内存。

以上问题的解决办法：
1、自己写字符转换函数，不要偷懒
Function that safely converts a 'WCHAR' String to 'LPSTR':
char* ConvertLPWSTRToLPSTR (LPWSTR lpwszStrIn)
{
  LPSTR pszOut = NULL;
  if (lpwszStrIn != NULL)
  {
 int nInputStrLen = wcslen (lpwszStrIn);

 // Double NULL Termination
 int nOutputStrLen = WideCharToMultiByte (CP_ACP, 0, lpwszStrIn, nInputStrLen, NULL, 0, 0, 0) + 2;
 pszOut = new char [nOutputStrLen];

 if (pszOut)
 {
   memset (pszOut, 0x00, nOutputStrLen);
   WideCharToMultiByte(CP_ACP, 0, lpwszStrIn, nInputStrLen, pszOut, nOutputStrLen, 0, 0);
 }
  }
  return pszOut;
}
等等一个一个的实现。

2、把字符转换部分放到一个函数中处理。

void fn2()
{
    USES_CONVERSION;
    DoSomething(A2W("SomeString"));
}

void fn()
{
    while(true)
    {
        fn2();
    }
}