struct InkPen
{
  void Write()
  {
    this->WriteImplementation();
  }

  void WriteImplementation()
  {
    std::cout << "Writing using a inkpen" << std::endl;
  }

};

struct BoldPen
{
  void Write()
  {
    std::cout << "Writing using a boldpen" << std::endl;
  }
};

template<class PenType>
class Writer : public PenType
{
public:
  void StartWriting()
  {
    PenType::Write();
  }
};

int main()
{
  Writer<InkPen> writer;
  writer.StartWriting();
  Writer<BoldPen> writer1;
  writer1.StartWriting();
  return 0;
}

I wrote the above code as part of learning policy based designs. I have few questions on the above code

1 - Does this implementation look correct? I mean: does it really look like a policy based design?

2 - I can now hook any kind of pens to writer. But what will I do when I got a pen with no default constructor (only parameterized constructors)? How will I handle this situation?

template<class PenType>
class Writer : public PenType
{
public:
  void StartWriting()
  {
    PenType::Write();
  }
};

3 - When the above code is used like

Writer<InkPen> writer;

I guess compiler will replace PenType with InkPen. If yes, why I am not able to call just Write() from StartWriting() instead of prefixing base class name (PenType::Write())?

4 - I think policy based design forces you to derive from classes which is semantically invalid. In the above code, a writer is derived from a pen only because writer uses a pen. But saying writer is a pen is semantically invalid. Is there any other better way to address this or I am missing something here?

Any thoughts?

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