Matching-Polymorphic-Types.cpp

#include "matchit.h"
#include <iostream>
#include <tuple>

struct Shape
{
  virtual ~Shape() = default;
};
struct Circle : Shape
{
  int radius;
};
struct Rectangle : Shape
{
  int width, height;
};

template <size_t I>
constexpr auto &get(Circle const &c)
{
  if constexpr (I == 0)
  {
    return c.radius;
  }
}

template <size_t I>
constexpr auto &get(Rectangle const &r)
{
  if constexpr (I == 0)
  {
    return r.width;
  }
  else if constexpr (I == 1)
  {
    return r.height;
  }
}

namespace std
{
  template <>
  class tuple_size<Circle> : public std::integral_constant<size_t, 1>
  {
  };
  template <>
  class tuple_size<Rectangle> : public std::integral_constant<size_t, 2>
  {
  };
} // namespace std

double get_area(const Shape &shape)
{
  using namespace matchit;
  Id<int> r, w, h;
  return match(shape)(
      // clang-format off
        pattern | as<Circle>(ds(r))       = 3.14 * r * r,
        pattern | as<Rectangle>(ds(w, h)) = w * h
      // clang-format on
  );
}

int32_t main()
{
  auto c = Circle{};
  c.radius = 5;
  std::cout << get_area(c) << std::endl;

  return 0;
}