17 Language support library [support]

17.11 Comparisons [cmp]

17.11.2 Comparison category types [cmp.categories]

17.11.2.4 Class strong_­ordering [cmp.strongord]

1
#
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);
}
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constexpr operator partial_ordering() const noexcept;
2
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Returns: 
value == 0 ? partial_ordering::equivalent :
value < 0  ? partial_ordering::less :
             partial_ordering::greater
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constexpr operator weak_ordering() const noexcept;
3
#
Returns: 
value == 0 ? weak_ordering::equivalent :
value < 0  ? weak_ordering::less :
             weak_ordering::greater
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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;
4
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Returns: v.value @ 0 for operator@.
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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;
5
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Returns: 0 @ v.value for operator@.
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constexpr strong_ordering operator<=>(strong_ordering v, unspecified) noexcept;
6
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Returns: v.
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constexpr strong_ordering operator<=>(unspecified, strong_ordering v) noexcept;
7
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Returns: v < 0 ? strong_­ordering​::​greater : v > 0 ? strong_­ordering​::​less : v.