va_start() and object programming

Question

I'm trying to do an object implementation of va_list. It's complex to explain with word, so look at the code :

class auto_va
{
public:
    va(int n)
    {
        va_start(varg, n);
    }
    ~va()
    {
        va_end(varg);
    }

private:
    va_list varg;

public:
    va_list& get_va()
    {
        return varg;
   }
};

double sum(int number, ...)
{
    int result(0);

    auto_va varg(number);

    for (;number != 0; --number)
        result += va_arg(varg.get_va(), int);

    return result;
}

int main()
{
    std::cout << sum(6, 1, 5, 3, 4, 7, 8) << std::endl;
}

The goal is an automatic va_end when the end of the block is reached. The problem is that I can't use va_start in a fixed parameter function, so, the code above don't compile.

I know I'm mixing old-school outdated C and C++ object features, but the problem still strange. Is there an interesting explanation or something ?

Finally, is there a correct alternative to what I'm trying to do, or an alternative to C-style infinite parameters function?