Question

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);
}

first why i get this compile error that solved only when i remove the const

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&) [with _Tp = Student, _Alloc = std::allocator]’

second

i have anther problem

undefined reference to b::ok

Answer 1

Second first: b::ok has been declared, but not defined. Someplace, (preferably b.cpp), you need to add:

  int b::ok;

As for your first problem, _ID is const, it value cannot be changed — but, you never give it a value to start with. You have to assign it an initial value:

  protected:
      const int _ID = 1234;

Now, you really should defined _ID as well, as we did with b::ok, but since it is const, the compiler may let you get away without doing that (some conditions apply).