The CComBSTR constructor does not always allocate sufficient space in ATL (241857)

CAUSE

This problem is due to a bug in the A2WBSTR function in the Atlconv.h file:

inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
{
 USES_CONVERSION;
 BSTR str = NULL;
 int nConvertedLen= MultiByteToWideChar(_acp, 0, lp, nLen, NULL, NULL)-1;
 str = ::SysAllocStringLen(NULL, nConvertedLen);
 if (str != NULL)
 {
  MultiByteToWideChar(_acp, 0, lp, -1,
   str, nConvertedLen);
 }
 return str;
}

If the MultiByteToWideChar function's fourth argument is -1, the string is assumed to be NULL-terminated and the number of bytes returned will include the NULL character. The result of this function should not subtract 1 (one) if nLen does not equal -1; it should only do this if nLen is equal to -1.

RESOLUTION

There are two ways to correct this problem:

Method 1

Use one of the conversion macros to wrap the string passed into the constructor:

CComBSTR bstr(5, T2OLE(buf));

-or-

CComBSTR bstr(5, OLESTR("01234");

This method uses the CComBSTR(int nSize, LPCOLESTR sz) constructor, which calls ::SysAllocStringLen, and then returns a BSTR with the number of characters equal to nSize.

Method 2

Change the incorrect code in Atlconv.h for the A2WBSTR function:

Change the code in Atlconv.h for the A2WBSTR function from the following:

inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
{
  USES_CONVERSION;
  BSTR str = NULL;
  int nConvertedLen = MultiByteToWideChar(_acp, 0, lp, nLen, NULL, NULL)-1;
  str = ::SysAllocStringLen(NULL, nConvertedLen);
  if (str != NULL)
  {
    MultiByteToWideChar(_acp, 0, lp, -1, str, nConvertedLen);
  }
  return str;
}

to the following:

inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
{
   USES_CONVERSION;
   BSTR str = NULL;
   int nConvertedLen = MultiByteToWideChar(_acp, 0, lp,
     nLen, NULL, NULL);
 
   // BUG FIX #1 (from Q241857): only subtract 1 from 
   // the length if the source data is nul-terminated
   if (nLen == -1)
      nConvertedLen--;
 
   str = ::SysAllocStringLen(NULL, nConvertedLen);
   if (str != NULL)
   {
     MultiByteToWideChar(_acp, 0, lp, nLen, str, nConvertedLen);
   }
   return str;
}

Save Atlconv.h.
On the Build menu, click Rebuild All. You need to do this on a Debug or ReleaseMinDependency build so that the modified code above is linked into your code.