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

This looks like a nice example of policy-based smart pointer implementation: link. Andrei Alexandrescu describes policy-based smart pointer implementation in one of his books. As to your questions now. I have some experience in this stuff but not enough to take my words for granted:

Ad 1 & 4. I guess policy-based design is more about templates than inheritance. You write a template class and template arguments are policy classes, like that:

template
class Baz {
    // implementation goes here
};

Then you use methods from policy classes in your class:

void Baz::someMethod(int someArg) {
    FooPolicy::methodInit();
    // some stuff
    BarPolicy::methodDone();
}

I use static methods in this example because often policy doesn't require any state. If it does, you incorporate policy's state by composition, not by inheritance:

template
class Baz {
  private:
    FooPolicy::State fooState; // might require 'typename' keyword, I didn't
                               // actually tried this in any compiler
    // rest of the Baz class
};

Ad 2. You can write a template specialization - for a particular combination of main class and it's policies you can write a special version of any method or constructor, AFAIK:

template <>
Baz::Baz(someArgument)
   : fooState(someArgument)
{
    // stuff here
}

Hope it helps you a bit,

Mike