C++20 Spaceship Operator Notes

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C++20 Spaceship Operator (<=>)

  • Pre C++20 requires 6 oeprators for your class to support all kinds of comparisons
    • ==, !=, <, <=, >, =>
    • Although mostly all just based on == or >, a hassle!
  • In C++20
    • == implies !=, so now only have to define ==. a != b now tries a != b, !(a == b) and !(b == a)
    • No equivalent for the other operators, but now have <=>, which is the only operator, other than ==, that you now need to define! Don't even need to define ==!
      • x <=> y == 0 when x and y are equal.
      • x <=> y < 0 when x < y.
      • x <=> y > 0 when x > y.
      • Thus, x <= y, can for example, be re-written as (x <=> y) <= 0, because if x == y, (x <=> y) will be 0, and if x < y, (x <=> y) will be less than zero.
    • Comparison categories/ordering
      • Strong (or total) ordering. For every value of type T, that value can compared with every other value of type T as being less than, equal to or greater than.
      • Weak ordering. Same as strong except equality is replace with equivalence. Equivalent types do not have to be equal. E.g. "JEHTECH" is equivalent to "jehtech" but not equal.
      • Patial ordering. It may not be possible to to specify an order between any two values.
    • The return value of <=> should sepcify the comparison category/ordering. For example 
      std::strong_ordering operator<=>(Type x, OtherType y) {...} // OR...
std::weak_ordering operator<=>(Type x, OtherType y) {...}   // OR...
std::partial_ordering operator<=>(Type x, OtherType y) {...}
    • The return value is std::ORDERING_TYPE::less, std::ORDERING_TYPE::equal, or std::ORDERING_TYPE::greater.
    • May find it useful, in your implementation to use if constexpr (requires { some-func-or-property <=> other.some-func-or-property(); }) { when trying to determine, at compile time, whether the types contained in your class/container themselves support the spaceship operator.