How to set a default implement for a member function? about proxy HOT 1 CLOSED

import std;

#include "proxy.h"

int Area_default() const {
  throw std::runtime_error("No Implemented"); 
}

// Specifications of abstraction
PRO_DEF_MEM_DISPATCH(MemDraw, Draw);
PRO_DEF_MEM_DISPATCH(MemArea, Area, Area, Area_default);

struct Drawable : pro::facade_builder
    ::add_convention<MemDraw, 
                    void(std::ostream& output, double custom_num),
                    void(std::ostream& output, int custom_num)>
    ::add_convention<MemArea, 
                    int() const>
    ::build {};

// Implementation
template<typename T>
class Rectangle {
 public:
  void Draw(std::ostream& out, T custom) const
      { out << "{Rectangle: width = " << width_ << ", height = " << height_ << " custom = " << custom <<"}"; }
  void SetWidth(T width) { width_ = width; }
  void SetHeight(T height) { height_ = height; }
  // T Area() const { return width_ * height_; }

 private:
  T width_;
  T height_;
};

// Client - Consumer
std::string PrintDrawableToString(pro::proxy<Drawable> p) {
  std::stringstream result;
  int custom_num = 10;
  result << "shape = ";
  p->Draw(result, custom_num);  // Polymorphic call
  result << ", area = " << p->Area();  // Polymorphic call
  return std::move(result).str();
}

// Client - Producer
pro::proxy<Drawable> CreateRectangleAsDrawable(int width, int height) {
  Rectangle<int> rect;
  rect.SetWidth(width);
  rect.SetHeight(height);
  pro::proxy<Drawable> p = &rect;
  return p;
}

int main() {
  auto obj = CreateRectangleAsDrawable(2,2);
  std::cout << PrintDrawableToString(std::move(obj)) << std::endl;
}