How do I use _malloca instead of _alloca in Win32 C++ project? - c++

Question

I'm updating an old C++ DLL project. For one of the exported functions there's
BSTR __stdcall j2cs( const long lJulian, int bDMY, BSTR sDelim ) {
USES_CONVERSION;
int iDay, iMonth;
long lYear;
char chDate[20];
char chInt[10];
char * cDelim = W2A( sDelim );
The W2A macro is defined as
#define W2A(lpw) (\
((_lpw = lpw) == NULL) ? NULL : (\
(_convert = (static_cast<int>(wcslen(_lpw))+1), \
(_convert>INT_MAX/2) ? NULL : \
ATLW2AHELPER((LPSTR) alloca(_convert*sizeof(WCHAR)), _lpw, _convert*sizeof(WCHAR), _acp))))
Visual Studio 2019 flags the W2A macro with the following compiler warning
Warning C6255 _alloca indicates failure by raising a stack overflow exception. Consider using _malloca instead.
How would I make the suggested change to the W2A macro? Or should I just ignore the warning?
LATER
My USES_CONVERSION macro is defined as
#define USES_CONVERSION int _convert = 0; (_convert); UINT _acp = ATL::_AtlGetConversionACP() /*CP_THREAD_ACP*/; (_acp); LPCWSTR _lpw = NULL; (_lpw); LPCSTR _lpa = NULL; (_lpa)

Answer 1

W2A can't be changed to using malloc. You have to add free in all places where W2A used. The ideal alternative is std::vector in place of _alloca.
Update the macro USES_CONVERSION, so it contains std::vector<WCHAR> _buffer; and update the macro W2A:
#define USES_CONVERSION int _convert = 0; (_convert); std::vector<WCHAR> _buffer; (_buffer); UINT _acp = ATL::_AtlGetConversionACP() /*CP_THREAD_ACP*/; (_acp); LPCWSTR _lpw = NULL; (_lpw); LPCSTR _lpa = NULL; (_lpa)
#define W2A(lpw) (\
((_lpw = lpw) == NULL) ? NULL : (\
(_convert = (static_cast<int>(wcslen(_lpw))+1), \
(_convert>INT_MAX/2) ? NULL : \
(_buffer.resize(_convert), \
ATLW2AHELPER((LPSTR) _buffer.data(), _lpw, _convert*sizeof(WCHAR), _acp)))))