title: static members and consts
body: <pre><code>class a
  {
  protected:
    const int _ID;
  
  public:
    a::a(int id){};
    a::top(int num);
  };
  
  class b : public a
  {
    static int ok;
    b::b(int id):a(id){};
    a::top(ok);
  }
  
  int main()
  {
    int t=5;
    b opj=b(t);
  }
  </code></pre>
  
  <p>first why i get this compile error that solved only when i remove the const</p>
  
  <p>non-static const member ‘const int Student::_ID’, can't use default assignment operator - 
   instantiated from ‘void std::vector::_M_insert_aux(__gnu_cxx::__normal_iterator, const _Tp&amp;) [with _Tp = Student, _Alloc = std::allocator]’</p>
  
  <p>second</p>
  
  <p>i have anther problem</p>
  
  <p>undefined reference to b::ok</p>