multi_dispatch.cpp

struct circle_t;
struct triangle_t;
struct square_t;

struct shape_t {
  using visitor_t = vdsp::visitor_t<circle_t, triangle_t, square_t>;
  virtual void accept(const visitor_t&) const = 0;
  virtual void do_thing()               const = 0;
};

struct circle_t : shape_t {
  void accept(const visitor_t& visitor) const override { visitor(*this); }
  void do_thing()                       const override { }
};

struct triangle_t : shape_t {
  void accept(const visitor_t& visitor) const override { visitor(*this); }
  void do_thing()                       const override { }
};

struct square_t : shape_t {
  void accept(const visitor_t& visitor) const override { visitor(*this); }
  void do_thing()                       const override { }
};

struct bar;
struct foobar;
struct foo {
  using visitor_t = vdsp::visitor_t<bar, foobar>;

  virtual void accept(const visitor_t&) const = 0;
};

struct bar : foo {
  void accept(const visitor_t& visitor) const override { visitor(*this); }
};

struct foobar : foo {
  void accept(const visitor_t& visitor) const override { visitor(*this); }
};

struct thingy {
  bool operator()(const circle_t&,   const bar&)    const { return true; }
  bool operator()(const circle_t&,   const foobar&) const { return false; }
  bool operator()(const triangle_t&, const bar&)    const { return true; }
  bool operator()(const triangle_t&, const foobar&) const { return true; }
  bool operator()(const square_t&,   const bar&)    const { return true; }
  bool operator()(const square_t&,   const foobar&) const { return true; }
};

bool do_thingy(const shape_t& shape, const foo& f) {
  return match<bool>(shape, f, thingy{});
}

int main() {
    circle_t circle;
    foobar f;
    std::cout<<std::boolalpha<<do_thingy(circle, f)<<'\n';
}