Re: Virtual Instancing of Derived Class

Hi. I've got a hierarchy of classes and I'm storing pointers to a
mid- level class Der1 in a vector. These pointers point to a handful
of classes derived from Der1, and Der2 is one such class. Classes
below Der1 implement a virtual function Helper() from the Base class,
and this function is typically called from a base class method
GetInstance(). I'm getting an error from g++ that I don't fully
understand.

The code is:

--------------------------------------------
#include <iostream>
#include <vector>

class Base {
  public:
     virtual Base *GetInstance() {
        std::cout << "in Base::GetInstance()" << std::endl;
        return Helper();
     }
     virtual Base *Helper() = 0;
};

class Der1 : public Base {
  public:
};

class Der2 : public Der1 {
  public:
     Der2 *Helper() {
        std::cout << "in Der2::Helper()" << std::endl;
        return new Der2();
     }
};

int main() {

  Der2 d2;
  Der1 *d1 = &d2;

  std::vector<Der1 *> der1s;

  der1s.push_back(d1->GetInstance());

  return 0;

}
--------------------------------------------

On compiling, g++ tells me:

woo.cpp: In function 'int main()':
woo.cpp:32: error: invalid conversion from 'Base*' to 'Der1*'
woo.cpp:32: error: initializing argument 1 of 'void std::vector<_Tp,
_Alloc>::push_back(const _Tp&) [with _Tp = Der1*, _Alloc =
std::allocator<Der1*>]'

It seems that the return value of Der2::Helper() is implicitly cast
from Der2 * to Base * by Base::GetInstance(). In order to fix this,
it seems I have to use dynamic_cast:

  der1s.push_back(dynamic_cast<Der1 *>(d1->GetInstance()));

Is this the correct diagnosis and a suitable solution to this problem?