Sakura Sakura - 4 months ago 19
C++ Question

How to calculate the size needed for formated wchar array, `CRT_SECURE` way?

I need to calculate the size need for formated wchar array.

Consider following code:

wchar_t* Format(const wchar_t* format, ...)
{
va_list args;
va_start(args, format);

wchar_t *w = NULL;
int len = _vsnwprintf(NULL, 0, format, args) + 1; // <------ Error
if (len > 0)
{
w = new wchar_t[len];
w[0] = 0;
_vsnwprintf_s(w, len, len, format, args);
}
va_end(args);

return w;
}


if
_CRT_SECURE_NO_WARNINGS
is turned on, above code work, if not, the error say:


'_vsnwprintf': This function or variable may be unsafe. Consider using
_vsnwprintf_s instead.


But I can not manage to use
_vsnwprintf_s
to calculate the size needed for the wchar buffer.

Following is what I tried:

int len = _vsnwprintf_s(NULL, 0, 0, format, args); // return 1

// debug assertion failed: string != NULL && DstSizeInWords > 0
int len = _vsnwprintf_s(NULL, 0, INT_MAX, format, args);

// debug assertion failed: string != NULL && DstSizeInWords > 0
int len = _vsnwprintf_s(NULL, INT_MAX, 0, format, args);

// debug assertion failed: string != NULL && DstSizeInWords > 0
int len = _vsnwprintf_s(NULL, INT_MAX, INT_MAX, format, args);


Other function:
_snwprintf
give
_CRT_SECURE
error and
_snwprintf_s
have same effect with
_vsnwprintf_s


Question:



Although I pretty okay with
_CRT_SECURE_NO_WARNINGS
turned on, I wonder how can I calculate the size needed for formated wchar array,
CRT_SECURE
way? Note that it must accept
va_list
as agrument.

VTT VTT
Answer Source

You should be able to obtain required buffer size using _vscwprintf:

_Check_return_ _Ret_z_ wchar_t *
Format(_Printf_format_string_ _In_z_ wchar_t const * const psz_format, ...)
{
    size_t formatted_string_length;
    va_list args;
    {
        va_start(args, psz_format);
        int const result(::_vscwprintf(psz_format, args));
        va_end(args);
        if(-1 == result)
        {
            ::abort();
        }
        assert(0 <= result);
        formatted_string_length = static_cast< size_t >(result);
    }
    size_t const buffer_size(formatted_string_length + 1u);
    wchar_t * const p_buffer(new wchar_t[buffer_size]);
    {
        va_start(args, psz_format);
        int const result(::_vsnwprintf_s(p_buffer, buffer_size, formatted_string_length, psz_format, args));
        va_end(args);
        if(-1 == result)
        {
            ::abort();
        }
        assert(0 <= result);
        assert(static_cast< size_t >(result) == formatted_string_length);
        assert(L'\0' == p_buffer[formatted_string_length]);
    }
    return p_buffer;
}