Perils of using W2A

Ever wondered why W2A leaks a lot of memory when used in a loop?

for(int i=0;i<count;i++)

{

      str= W2A(bstr);

      cout<<str<<endl;

           

}

When we try and step in at W2A we end up with following ASM code.

*********************************************************************************

_alloca_probe_16 proc                   ; 16 byte aligned alloca

        push    ecx

        lea     ecx, [esp] + 8          ; TOS before entering this function

        sub     ecx, eax                ; New TOS

        and     ecx, (16 - 1)           ; Distance from 16 bit align (align down)

        add     eax, ecx                ; Increase allocation size

        sbb     ecx, ecx                ; ecx = 0xFFFFFFFF if size wrapped around

        or      eax, ecx                ; cap allocation size on wraparound

        pop     ecx                     ; Restore ecx

        jmp     _chkstk

***********************************************************************************

What we see here is the stack is resized as in, W2A does an allocation on the stack
      so what would happen when this goes in a loop? obvious answer would be, the stack
      grows, and ends up with Stack Overflow problem.

So we end up with a couple of questions..

1.What is the best way takle this problem.

2.Are there any alternatives.
      3.Why USES_CONVERSION MACRO when we use W2A.

1.One way is to manually convert it CString like this function MyW2A
  does.

CString MyW2A(BSTR bstr)

{

      CString s;

      If(bstr!=NULL)

{

      LPSTR p = s.GetBuffer(SysStringLen(bstr) + 1);

      ::WideCharToMultiByte(CP_ACP,           

                                      0,                

                                      bstr,                 

                                      -1,                // assume NUL-terminated

                                      p,                

                                      SysStringLen(bstr)+1,

                                      NULL,             

                                      NULL);            

      s.ReleaseBuffer();

      }

      return s;

}//MyW2A
     

JNM(Joseph NewComer) has a very good article on String management here

http://www.flounder.com/cstring.htm

      2. CW2A which has been provided as part of ATL 7.0 can be used instead of W2A,
                another cool thing about this is, USES_CONVERSION is not required for CW2A.

3. Why do we need, USES_CONVERSION MACRO when we use W2A?

   The answer lies in Atlconv.h file and the way these macros have been defined.

  

/*
*where is the _lpw defined?
*where is _convert defined?
*where is _acp defined?
*/

   #define W2A(lpw) (\

      ((_lpw = lpw) == NULL) ? NULL : (\

            (_convert = (lstrlenW(_lpw)+1), \

            (_convert>INT_MAX/2) ? NULL : \

            ATLW2AHELPER((LPSTR) alloca(_convert*sizeof(WCHAR)), _lpw, 
   _convert*sizeof(WCHAR), _acp))))

/*

* U guessed it right? Look at the definition of USES_CONVERSION
* Simple right!!!
*/

#define USES_CONVERSION int _convert = 0; (_convert);\
UINT _acp = ATL::_AtlGetConversionACP() /*CP_THREAD_ACP*/;\
(_acp); LPCWSTR _lpw = NULL; (_lpw); \
LPCSTR _lpa = NULL; (_lpa)

Another clue would be when we compile without the USES_CONVERSION macro, we end up with the following errors.

1>c:\..\vc days\.....\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_lpw' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_convert' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_lpw' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_convert' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_convert' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_lpw' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_convert' : undeclared identifier

1>c:\..\vc days\......\w2atest\w2atest\w2atest.cpp(45) : error C2065: '_acp' : undeclared identifier