Policy based design and best practices - C++

Question

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

  void WriteImplementation()
  {
    std::cout << \"Writing using a inkpen\" <&l         


        

Answer 1

1 - Is this implementation looks correct? I mean is it really looks like a policy based design?

Policy classes derive their usefulness from combining behaviors to produce a rich variety of combinations. When you have a single template parameter like this, it's not much of a policy class.

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?

Again, this is an odd example of a policy class. However, to directly answer your question, you can provide a constructor which accepts PenType. You should probably also avoid inheriting from PenType and store it as a member instead (no need to tightly couple your policy class with its policies).

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())?

When you inherit from a class template, you have to specify this->member or BaseClass::member.

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?

Store PenType as a member as suggested above. Always prefer composition to inheritance as it avoids the tight coupling relationship of inheritance.