cpp-opaque-typedef/opaque_typedef.hpp

//
//  opaque_typedef.hpp
//  opaque-type
//
//  Inspired by https://foonathan.github.io/blog/2016/10/19/strong-typedefs.html and others
//
//  Created by Sam Jaffe on 8/16/16.
//

#pragma once

#include <type_traits>

namespace types { namespace detail {
  template <typename FromOpaque>
  struct is_a {
    FromOpaque const & self() const {
      return *(FromOpaque const *)(void const *)(this);
    }
  };
}}

namespace types {
  template <typename Base, typename Tag, template <typename> class... Skills>
  class opaque_typedef : public Skills<opaque_typedef<Base, Tag, Skills...>>... {
  public:
    using super = opaque_typedef;
    
  private:
    Base value_;
    
  public:
    opaque_typedef() = default;
    explicit opaque_typedef(Base const & value) : value_(value) {};
    explicit opaque_typedef(Base && value) : value_(std::forward<Base>(value)) {};
    
    Base const & get() const { return value_; }
    explicit operator Base const &() const { return value_; }
  };
  
  template <typename Opaque>
  struct EqualityComparable {
    friend bool operator==(Opaque const & lhs, Opaque const & rhs) {
      return lhs.get() == rhs.get();
    }
    friend bool operator!=(Opaque const & lhs, Opaque const & rhs) {
      return !(lhs == rhs);
    }
  };
  
  template <typename Opaque>
  struct Comparable : EqualityComparable<Opaque> {
    friend bool operator<(Opaque const & lhs, Opaque const & rhs) {
      return lhs.get() < rhs.get();
    }
    friend bool operator>(Opaque const & lhs, Opaque const & rhs) {
      return rhs.get() < lhs.get();
    }
    friend bool operator<=(Opaque const & lhs, Opaque const & rhs) {
      return !(rhs.get() < lhs.get());
    }
    friend bool operator>=(Opaque const & lhs, Opaque const & rhs) {
      return !(lhs.get() < rhs.get());
    }
  };
  
  template <typename ToOpaque>
  struct Convertable {
    template <typename FromOpaque>
    class type : detail::is_a<FromOpaque> {
    public:
      operator ToOpaque() const;
    };
  };
}