17 Language support library [language.support]

17.11 Comparisons [cmp]

17.11.2 Comparison category types [cmp.categories]

17.11.2.1 Preamble [cmp.categories.pre]

The types partial_­ordering, weak_­ordering, and strong_­ordering are collectively termed the comparison category types.
Each is specified in terms of an exposition-only data member named value whose value typically corresponds to that of an enumerator from one of the following exposition-only enumerations:
enum class eq { equal = 0, equivalent = equal,
                nonequal = 1, nonequivalent = nonequal };   // exposition only
enum class ord { less = -1, greater = 1 };                  // exposition only
enum class ncmp { unordered = -127 };                       // exposition only
Note
:
The type strong_­ordering corresponds to the term total ordering in mathematics.
— end note
 ]
The relational and equality operators for the comparison category types are specified with an anonymous parameter of unspecified type.
This type shall be selected by the implementation such that these parameters can accept literal 0 as a corresponding argument.
Example
:
nullptr_­t meets this requirement.
— end example
 ]
In this context, the behavior of a program that supplies an argument other than a literal 0 is undefined.
For the purposes of subclause [cmp.categories], substitutability is the property that f(a) == f(b) is true whenever a == b is true, where f denotes a function that reads only comparison-salient state that is accessible via the argument's public const members.

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.217
namespace std {
  class partial_ordering {
    int value;          // exposition only
    bool is_ordered;    // exposition only

    // exposition-only constructors
    constexpr explicit
      partial_ordering(eq v) noexcept : value(int(v)), is_ordered(true) {}      // exposition only
    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(eq::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.

17.11.2.3 Class weak_­ordering [cmp.weakord]

The weak_­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]), and (b) does not imply substitutability.
namespace std {
  class weak_ordering {
    int value;  // exposition only

    // exposition-only constructors
    constexpr explicit weak_ordering(eq v) noexcept : value(int(v)) {}  // exposition only
    constexpr explicit weak_ordering(ord v) noexcept : value(int(v)) {} // exposition only

  public:
    // valid values
    static const weak_ordering less;
    static const weak_ordering equivalent;
    static const weak_ordering greater;

    // conversions
    constexpr operator partial_ordering() const noexcept;

    // comparisons
    friend constexpr bool operator==(weak_ordering v, unspecified) noexcept;
    friend constexpr bool operator==(weak_ordering v, weak_ordering w) noexcept = default;
    friend constexpr bool operator< (weak_ordering v, unspecified) noexcept;
    friend constexpr bool operator> (weak_ordering v, unspecified) noexcept;
    friend constexpr bool operator<=(weak_ordering v, unspecified) noexcept;
    friend constexpr bool operator>=(weak_ordering v, unspecified) noexcept;
    friend constexpr bool operator< (unspecified, weak_ordering v) noexcept;
    friend constexpr bool operator> (unspecified, weak_ordering v) noexcept;
    friend constexpr bool operator<=(unspecified, weak_ordering v) noexcept;
    friend constexpr bool operator>=(unspecified, weak_ordering v) noexcept;
    friend constexpr weak_ordering operator<=>(weak_ordering v, unspecified) noexcept;
    friend constexpr weak_ordering operator<=>(unspecified, weak_ordering v) noexcept;
  };

  // valid values' definitions
  inline constexpr weak_ordering weak_ordering::less(ord::less);
  inline constexpr weak_ordering weak_ordering::equivalent(eq::equivalent);
  inline constexpr weak_ordering weak_ordering::greater(ord::greater);
}
constexpr operator partial_ordering() const noexcept;
Returns:
value == 0 ? partial_ordering::equivalent :
value < 0  ? partial_ordering::less :
             partial_ordering::greater
constexpr bool operator==(weak_ordering v, unspecified) noexcept; constexpr bool operator< (weak_ordering v, unspecified) noexcept; constexpr bool operator> (weak_ordering v, unspecified) noexcept; constexpr bool operator<=(weak_ordering v, unspecified) noexcept; constexpr bool operator>=(weak_ordering v, unspecified) noexcept;
Returns: v.value @ 0 for operator@.
constexpr bool operator< (unspecified, weak_ordering v) noexcept; constexpr bool operator> (unspecified, weak_ordering v) noexcept; constexpr bool operator<=(unspecified, weak_ordering v) noexcept; constexpr bool operator>=(unspecified, weak_ordering v) noexcept;
Returns: 0 @ v.value for operator@.
constexpr weak_ordering operator<=>(weak_ordering v, unspecified) noexcept;
Returns: v.
constexpr weak_ordering operator<=>(unspecified, weak_ordering v) noexcept;
Returns: v < 0 ? weak_­ordering​::​greater : v > 0 ? weak_­ordering​::​less : v.

17.11.2.4 Class strong_­ordering [cmp.strongord]

The strong_­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]), and (b) does imply substitutability.
namespace std {
  class strong_ordering {
    int value;  // exposition only

    // exposition-only constructors
    constexpr explicit strong_ordering(eq v) noexcept : value(int(v)) {}    // exposition only
    constexpr explicit strong_ordering(ord v) noexcept : value(int(v)) {}   // exposition only

  public:
    // valid values
    static const strong_ordering less;
    static const strong_ordering equal;
    static const strong_ordering equivalent;
    static const strong_ordering greater;

    // conversions
    constexpr operator partial_ordering() const noexcept;
    constexpr operator weak_ordering() const noexcept;

    // comparisons
    friend constexpr bool operator==(strong_ordering v, unspecified) noexcept;
    friend constexpr bool operator==(strong_ordering v, strong_ordering w) noexcept = default;
    friend constexpr bool operator< (strong_ordering v, unspecified) noexcept;
    friend constexpr bool operator> (strong_ordering v, unspecified) noexcept;
    friend constexpr bool operator<=(strong_ordering v, unspecified) noexcept;
    friend constexpr bool operator>=(strong_ordering v, unspecified) noexcept;
    friend constexpr bool operator< (unspecified, strong_ordering v) noexcept;
    friend constexpr bool operator> (unspecified, strong_ordering v) noexcept;
    friend constexpr bool operator<=(unspecified, strong_ordering v) noexcept;
    friend constexpr bool operator>=(unspecified, strong_ordering v) noexcept;
    friend constexpr strong_ordering operator<=>(strong_ordering v, unspecified) noexcept;
    friend constexpr strong_ordering operator<=>(unspecified, strong_ordering v) noexcept;
  };

  // valid values' definitions
  inline constexpr strong_ordering strong_ordering::less(ord::less);
  inline constexpr strong_ordering strong_ordering::equal(eq::equal);
  inline constexpr strong_ordering strong_ordering::equivalent(eq::equivalent);
  inline constexpr strong_ordering strong_ordering::greater(ord::greater);
}
constexpr operator partial_ordering() const noexcept;
Returns:
value == 0 ? partial_ordering::equivalent :
value < 0  ? partial_ordering::less :
             partial_ordering::greater
constexpr operator weak_ordering() const noexcept;
Returns:
value == 0 ? weak_ordering::equivalent :
value < 0  ? weak_ordering::less :
             weak_ordering::greater
constexpr bool operator==(strong_ordering v, unspecified) noexcept; constexpr bool operator< (strong_ordering v, unspecified) noexcept; constexpr bool operator> (strong_ordering v, unspecified) noexcept; constexpr bool operator<=(strong_ordering v, unspecified) noexcept; constexpr bool operator>=(strong_ordering v, unspecified) noexcept;
Returns: v.value @ 0 for operator@.
constexpr bool operator< (unspecified, strong_ordering v) noexcept; constexpr bool operator> (unspecified, strong_ordering v) noexcept; constexpr bool operator<=(unspecified, strong_ordering v) noexcept; constexpr bool operator>=(unspecified, strong_ordering v) noexcept;
Returns: 0 @ v.value for operator@.
constexpr strong_ordering operator<=>(strong_ordering v, unspecified) noexcept;
Returns: v.
constexpr strong_ordering operator<=>(unspecified, strong_ordering v) noexcept;
Returns: v < 0 ? strong_­ordering​::​greater : v > 0 ? strong_­ordering​::​less : v.