Std :: String的C ++ Printf样式格式

Question

I was writing a utility function to do sprintf like formatting for std::string or std::wstring based on Unicode settings.

#ifdef _UNICODE
typedef std::wstring MyString ;
typedef TCHAR MyChar ;
#define MYTEXT TEXT
#else
typedef std::string MyString ;
typedef char MyChar ;
#define MYTEXT
#endif //_UNICODE

MyString Utils::GetStringPrintf(const MyString kstrText, ...)
{
    int iCharsWritten = 0 ;
    MyString strFinal ;
    MyChar szBufferTouse[8194] ; //Hopefully Long enough
    va_list     fmtList ;
    va_start(fmtList, kstrText) ;
    /*int iVal = va_arg(fmtList, int) ;*/ =>Just for debugging

#ifdef _UNICODE
    iCharsWritten = _stprintf_s(szBufferTouse,  8194, kstrText.c_str(), fmtList) ;
#else
    iCharsWritten = sprintf_s(szBufferTouse, 8194, kstrText.c_str(), fmtList) ;
#endif //_UNICODE

    va_end(fmtList) ;
    strFinal = szBufferTouse ;
    return strFinal ;
}

When called like :

int iId = 2 ;
MyString strFileName = Utils::GetStringPrintf(MYTEXT("Student_%d.png"), iId) ;  

//For Unicode am getting Student_1633600.png instead of Student_2.png //For non-Unicode am getting Student_1633800.png instead of Student_2.png

Upon debugging I do get the value of iVal as 2, but somehow the values get bad upon passing to sprintf.

However if I call, sprintf directly,

MyChar szFilename[200] ;
int iId = 2 ;
_stprintf_s(szFilename, 200, MYTEXT("Student_%d.png"), iId) ;

Am getting proper output, ie Student_2.png

I did refer other posts in Stack Overflow regarding sprintf functionality and found my code to be doing quite similar to those. Is there any problem in passing va_list repeatedly to functions.

Answer 1

Replace sprintf_s with vsprintf (and remove size) or with vsprintf_s if available.

Quoting from manpage of printf. (Emphasis mine)

The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to the functions printf(), fprintf(), sprintf(), snprintf() , respectively, except that they are called with a va_list instead of a variable number of arguments . These functions do not call the va_end macro. Because they invoke the va_arg macro, the value of ap is undefined after the call.