17 Language support library [support]

17.11 Comparisons [cmp]

17.11.2 Comparison category types [cmp.categories]

17.11.2.2 Class partial_­ordering [cmp.partialord]

The partial_­ordering type is typically used as the result type of a three-way comparison operator that (a) admits all of the six two-way comparison operators ([expr.rel], [expr.eq]), (b) does not imply substitutability, and (c) permits two values to be incomparable.216
namespace std { class partial_ordering { int value; // exposition only bool is_ordered; // exposition only // exposition-only constructors constexpr explicit partial_ordering(ord v) noexcept : value(int(v)), is_ordered(true) {} // exposition only constexpr explicit partial_ordering(ncmp v) noexcept : value(int(v)), is_ordered(false) {} // exposition only public: // valid values static const partial_ordering less; static const partial_ordering equivalent; static const partial_ordering greater; static const partial_ordering unordered; // comparisons friend constexpr bool operator==(partial_ordering v, unspecified) noexcept; friend constexpr bool operator==(partial_ordering v, partial_ordering w) noexcept = default; friend constexpr bool operator< (partial_ordering v, unspecified) noexcept; friend constexpr bool operator> (partial_ordering v, unspecified) noexcept; friend constexpr bool operator<=(partial_ordering v, unspecified) noexcept; friend constexpr bool operator>=(partial_ordering v, unspecified) noexcept; friend constexpr bool operator< (unspecified, partial_ordering v) noexcept; friend constexpr bool operator> (unspecified, partial_ordering v) noexcept; friend constexpr bool operator<=(unspecified, partial_ordering v) noexcept; friend constexpr bool operator>=(unspecified, partial_ordering v) noexcept; friend constexpr partial_ordering operator<=>(partial_ordering v, unspecified) noexcept; friend constexpr partial_ordering operator<=>(unspecified, partial_ordering v) noexcept; }; // valid values' definitions inline constexpr partial_ordering partial_ordering::less(ord::less); inline constexpr partial_ordering partial_ordering::equivalent(ord::equivalent); inline constexpr partial_ordering partial_ordering::greater(ord::greater); inline constexpr partial_ordering partial_ordering::unordered(ncmp::unordered); }
constexpr bool operator==(partial_ordering v, unspecified) noexcept; constexpr bool operator< (partial_ordering v, unspecified) noexcept; constexpr bool operator> (partial_ordering v, unspecified) noexcept; constexpr bool operator<=(partial_ordering v, unspecified) noexcept; constexpr bool operator>=(partial_ordering v, unspecified) noexcept;
Returns: For operator@, v.is_­ordered && v.value @ 0.
constexpr bool operator< (unspecified, partial_ordering v) noexcept; constexpr bool operator> (unspecified, partial_ordering v) noexcept; constexpr bool operator<=(unspecified, partial_ordering v) noexcept; constexpr bool operator>=(unspecified, partial_ordering v) noexcept;
Returns: For operator@, v.is_­ordered && 0 @ v.value.
constexpr partial_ordering operator<=>(partial_ordering v, unspecified) noexcept;
Returns: v.
constexpr partial_ordering operator<=>(unspecified, partial_ordering v) noexcept;
Returns: v < 0 ? partial_­ordering​::​greater : v > 0 ? partial_­ordering​::​less : v.
That is, a < b, a == b, and a > b might all be false.