Is this "upcast" legal?

Hi,

the following code snippet compiles fine and behaves as expected, but I
wonder whether that behavior is guaranteed by the standard:

#include <iostream>

struct base
{
   struct iterator
   {
     iterator(base & b) : _base(b) { }

     base & _base;
   };
};

struct derived : public base
{
   struct iterator : public base::iterator
   {
     iterator(derived & d) : base::iterator(d) { }
   };

   void print() { std::cout << "derived::print()" << std::endl; }
};

int main(int, char**)
{
   derived d;
   derived::iterator i( d );
   static_cast<derived &>(i._base).print();
   return 0;
}

Why does that static_cast from `base' to `derived' succeed? Would this
code still work in case multiple inheritance?

Best regards,
Peter

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