decorator.cpp

#include <iostream>
#include <tuple>
#include <type_traits>

namespace jc {

class Shape {
 public:
  virtual void Display() = 0;
  virtual ~Shape() = default;
};

template <template <typename> class... T>
class Line : public Shape, public T<Line<>>... {
 public:
  void Display() override {
    std::apply([](auto&&... x) { (x.Display(), ...); },
               std::forward_as_tuple(T<Line<>>()...));
    std::cout << "line\n";
  }
};

template <template <typename> class... T>
class Text : public Shape, public T<Text<>>... {
 public:
  void Display() override {
    std::apply([](auto&&... x) { (x.Display(), ...); },
               std::forward_as_tuple(T<Text<>>()...));
    std::cout << "text\n";
  }
};

template <typename T>
class ColorDecorator {
 public:
  void Display() {
    if constexpr (std::is_constructible_v<Line<>*, T*>) {
      std::cout << "red ";
    } else {
      std::cout << "black ";
    }
  }
};

template <typename T>
class WeightDecorator {
 public:
  void Display() {
    if constexpr (std::is_constructible_v<Line<>*, T*>) {
      std::cout << "bold ";
    } else {
      std::cout << "normal ";
    }
  }
};

}  // namespace jc

int main() {
  jc::Line line1;
  jc::Line<jc::ColorDecorator> line2;
  jc::Line<jc::WeightDecorator> line3;
  jc::Line<jc::ColorDecorator, jc::WeightDecorator> line4;

  line1.Display();  // line
  line2.Display();  // red line
  line3.Display();  // bold line
  line4.Display();  // red bold line

  jc::Text text1;
  jc::Text<jc::ColorDecorator> text2;
  jc::Text<jc::WeightDecorator> text3;
  jc::Text<jc::ColorDecorator, jc::WeightDecorator> text4;

  text1.Display();  // text
  text2.Display();  // black text
  text3.Display();  // normal text
  text4.Display();  // black normal text
}