issues with const and iterator

i am using some code from a library in which i do not want to edit.
building on the library i have some compile time errors about
assigning iterators. below is the distilled code and error message.
appreciate your help:

  1 #include <vector>
  2 // --------------------------------------
  3 // struct B cannot be edited
  4 // --------------------------------------
  5 template<class D>
  6 struct B {
  7
  8 B(): data(5) { }
  9 const D& get() const { return data; }
 10 D data;
 11 };
 12 // --------------------------------------
 13 // edit struct C to make the code compile
 14 // --------------------------------------
 15 struct C {
 16
 17 typedef std::vector<int> D;
 18 typedef D::iterator Ditr;
 19
 20 C () {
 21 bd = new B<D>();
 22 begin_itr = (bd->get()).begin(); // error happens here
 23 }
 24 Ditr begin_itr;
 25 B<D> * bd;
 26 };
 27 // --------------------------------------
 28 int main ( ) { }

g++ -Wall vector_t.C
vector_t.C: In constructor =91C::C()':
vector_t.C:22: error: no match for =91operator=' in =91((C*)this)-


((C*)this)->C::bd-> B<D>::get [with D = std::vector<int,
std::allocator<int> >]())->std::vector<_Tp, _Alloc>::begin [with _Tp =
int, _Alloc = std::allocator<int>]()'
/usr/include/c++/4.2.1/bits/stl_iterator.h:637: note: candidates are:
__gnu_cxx::__normal_iterator<int*, std::vector<int,
std::allocator<int> > >& __gnu_cxx::__normal_iterator<int*,
std::vector<int, std::allocator<int> > >::operator=(const
__gnu_cxx::__normal_iterator<int*, std::vector<int,
std::allocator<int> > >&)