how to use std::rel_ops to supply comparison operators automatically? [duplicate]

Answer 1

I'd use the <boost/operators.hpp> header :

#include <boost/operators.hpp>

struct S : private boost::totally_ordered<S>
{
  bool operator<(const S&) const { return false; }
  bool operator==(const S&) const { return true; }
};

int main () {
  S s;
  s < s;
  s > s;
  s <= s;
  s >= s;
  s == s;
  s != s;
}

---

Or, if you prefer non-member operators:

#include <boost/operators.hpp>

struct S : private boost::totally_ordered<S>
{
};

bool operator<(const S&, const S&) { return false; }
bool operator==(const S&, const S&) { return true; }

int main () {
  S s;
  s < s;
  s > s;
  s <= s;
  s >= s;
  s == s;
  s != s;
}

Answer 2

Actually only < should suffice. Do it like so:

a == b <=> !(a<b) && !(b<a)

a > b <=> b < a

a <= b <=> !(b<a)

a != b <=> (a<b) || (b < a)

And so on for symmetric cases.

Answer 3

> equals !(<=)
>= equals !(<)
<= equals == or <
!= equals !(==)

Something like this?