cpp-opaque-typedef/include/opaque_typedef/convertable.hpp

#pragma once

namespace types {
  
  template <typename To>
  struct Convertable {
    template <typename Self>
    class type {
    private:
      Self const & self() const { return *(Self const *)(void const *)(this); }
    public:
      operator To() const;
    };
  };
  
}