Question

I need to create a function to create a set of integers. As I don’t know the no. of arguments, I thought of using the ellipses.

   void f1(...)
   {
     va_list ap;
       //how to initialize ap as I don't know the last actual argument as there is no such argument!!!
   }

Also, is there any other way to know whether the list has ended instead of supplying a last argument with a value that denotes the end of list????

Please help!!!

Answer 1

Let’s rewrite your function a little.

Set &f1(int a1, ...)
{
    Set &new_set = new(Set);
    new_set.add(a1);
    va_list args;
    va_start(args, a1);
    int aN;
    while ((aN = va_arg(args, int)) != -1)
        new_set.add(aN);
    va_end(args);
    return new_set;
}

Now, this can be invoked with:

Set s1 = f1(1, -1);
Set s2 = f1(1, 2, -1);
Set s3 = f1(1, 2, 3, -1);

Note, though, that the number of arguments is known at each call site. An alternative interface design specifies the number of parameters in the first argument:

Set s1 = f1(1, 1);
Set s2 = f1(2, 1, 2);
Set s3 = f1(3, 1, 2, 3);

However, if you need to specify an arbitrary number of arguments in a single call, then you need an interface more like:

Set &f1(size_t num, const int *array);

This allows you to specify the number of items in the array.