Problem with static downcast of base type to derived type

I have a program which crashes when:

1 - I use static_cast to turn a base type pointer into a pointer to a
derived type

2 - I use this new pointer to call a function in an object of the
derived type

3 - this function then 'grows' the derived type object (by pushing
onto a vector).

Is this actually possible, or should I be doing something else? In
more detail, what I'm doing is (there's a complete test program below,
which crashes):

1 - I have a base class which doesn't do very much;

2 - I declare a derived class with public inheritance from the base
class;

3 - the derived class has a method which pushes onto a vector in the
derived class;

4 - I create an STL set of base class objects;

5 - I then insert an object of the derived class into this set. I use
the set 'insert' method, which returns an iterator to the new object
in the set, but this is an iterator to a *base* object (I hope)

6 - I convert the iterator to a pointer, and static_cast the pointer
to a pointer to an object of the derived class

7 - I call my new method via this pointer, pushing onto the vector in
the derived-class-object. Bang.

Any help much appreciated.

Thanks -

Dom

===========================================
#include <vector>
#include <set>
#include <iostream>

using std::vector;
using std::set;
using std::cout;

class A {
public:
   A(int v) :
         val(v) {}
   int val;
};

class D : public A {
public:
   D(int v) :
         A(v) {}

   mutable vector<int> vec;

   void dfunc(void) const {
      std::cout << "hello world " << val << "!\n";
      // ********************************************
      // Ok without this line, crashes with this line
      vec.push_back(val);
      // ********************************************
   }
};

class ALess {
public :
   bool operator() (const A& a1, const A& a2) const {
      return a1.val < a2.val;
   }
};

typedef set<A, ALess> ASet;
typedef ASet::iterator ASetIter;
typedef std::pair<ASetIter, bool> ASetInsRetVal;

int main() {
   ASetInsRetVal retval;
   ASet aset;

   for(int i=0; i<4; i++) {
      retval = aset.insert(D(i));
      if(!retval.second)
         cout << "insert failed\n";
      else {
         const D *dptr = static_cast<const D*>(&(*retval.first));
         dptr->dfunc(); // crashes - see above
      }
   }
}
==============================================