comparable.h

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#ifndef META_COMPARABLE_H_
#define META_COMPARABLE_H_

namespace meta
{
namespace util
{

template <class Derived>
class comparable
{
  public:
    friend bool operator==(const comparable& lhs, const comparable& rhs)
    {
        return !(lhs.as_derived() < rhs.as_derived())
               && !(rhs.as_derived() < lhs.as_derived());
    }

    friend bool operator!=(const comparable& lhs, const comparable& rhs)
    {
        return !(lhs == rhs);
    }

    friend bool operator>(const comparable& lhs, const comparable& rhs)
    {
        return rhs.as_derived() < lhs.as_derived();
    }

    friend bool operator<=(const comparable& lhs, const comparable& rhs)
    {
        return lhs.as_derived() < rhs.as_derived() || lhs == rhs;
    }

    friend bool operator>=(const comparable& lhs, const comparable& rhs)
    {
        return lhs > rhs || lhs == rhs;
    }

  private:
    inline const Derived& as_derived() const
    {
        return static_cast<const Derived&>(*this);
    }
};
}
}
#endif