1 Dynamic binding, polymorphism

class CounterBase {
 public:
  CounterBase() = default;
  virtual ~CounterBase() = default;

  virtual void Increment() { count_++; }
  int count() const { return count_; }

 protected:
  int count_ = 0;
};

class CounterDouble : public CounterBase {
 public:
  CounterDouble() : CounterBase() {}
  virtual ~CounterDouble() = default;
  void Increment() { count_ += 2; }
};

int main() {
  {
    CounterDouble counter_double;
    CounterBase& counter_base = counter_double;  // Polymorphism with reference.
    counter_base.Increment();
    cout << counter_base.count() << endl;  // 2.
  }

  {
    CounterDouble counter_double;
    CounterBase* counter_base = &counter_double;  // Polymorphism with pointer.
    counter_base->Increment();
    cout << counter_base->count() << endl;  // 2.
  }
  // If there is no virtual for the function in base class, there will be no polymorphism.
  // Dynamic binding condition:
  // (1) virtual function
  // (2) inherit
  // (3) assign derive type to base type with pointer or reference.
  return 0;
}

2 Curiously Recursive Template Pattern(CRTP)

template <typename Derived>
class CounterBase {
 public:
  CounterBase() = default;
  virtual ~CounterBase() = default;

  // CRTP: compile-time polymorphism, static binding, better performance than dynamic binding.
  // Define the interface, and derived class must implement them.
  void increment() { static_cast<Derived*>(this)->increment_impl(); }
  int count() const { return static_cast<const Derived*>(this)->count_impl(); }
};

class CounterDouble : public CounterBase<CounterDouble> {
 public:
  CounterDouble() : CounterBase<CounterDouble>() {}
  virtual ~CounterDouble() = default;
  void increment_impl() { count_ += 2; }
  int count_impl() const { return count_; }

 private:
  int count_ = 0;
};

int main() {
  {
    CounterDouble counter_double;
    CounterBase<CounterDouble>& counter_base = counter_double;  // CRTP
    counter_base.increment();
    cout << counter_base.count() << endl;  // 2.
  }

  {
    CounterDouble counter_double;
    CounterBase<CounterDouble>* counter_base = &counter_double;  // CRTP
    counter_base->increment();
    cout << counter_base->count() << endl;  // 2.
  }
  return 0;
}

3 Difference between them

• CRTP has better performance than dynamic binding polymorphism(spped: 10X faster in benchmark)
• But CRTP is not so flexible compared to dynamic binding polymorphism.