From Jason Turner

[cmp.weakord]

C++17 → C++20 3.5 KB 94 lines
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+ #### Class `weak_ordering` <a id="cmp.weakord">[[cmp.weakord]]</a>
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+
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+ The `weak_ordering` type is typically used as the result type of a
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+ three-way comparison operator [[expr.spaceship]] that (a) admits all of
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+ the six two-way comparison operators ([[expr.rel]], [[expr.eq]]), and
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+ (b) does not imply substitutability.
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+
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+ ``` cpp
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+ namespace std {
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+ class weak_ordering {
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+ int value; // exposition only
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+
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+ // exposition-only constructors
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+ constexpr explicit weak_ordering(eq v) noexcept : value(int(v)) {} // exposition only
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+ constexpr explicit weak_ordering(ord v) noexcept : value(int(v)) {} // exposition only
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+
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+ public:
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+ // valid values
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+ static const weak_ordering less;
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+ static const weak_ordering equivalent;
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+ static const weak_ordering greater;
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+
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+ // conversions
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+ constexpr operator partial_ordering() const noexcept;
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+
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+ // comparisons
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+ friend constexpr bool operator==(weak_ordering v, unspecified) noexcept;
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+ friend constexpr bool operator==(weak_ordering v, weak_ordering w) noexcept = default;
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+ friend constexpr bool operator< (weak_ordering v, unspecified) noexcept;
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+ friend constexpr bool operator> (weak_ordering v, unspecified) noexcept;
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+ friend constexpr bool operator<=(weak_ordering v, unspecified) noexcept;
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+ friend constexpr bool operator>=(weak_ordering v, unspecified) noexcept;
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+ friend constexpr bool operator< (unspecified, weak_ordering v) noexcept;
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+ friend constexpr bool operator> (unspecified, weak_ordering v) noexcept;
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+ friend constexpr bool operator<=(unspecified, weak_ordering v) noexcept;
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+ friend constexpr bool operator>=(unspecified, weak_ordering v) noexcept;
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+ friend constexpr weak_ordering operator<=>(weak_ordering v, unspecified) noexcept;
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+ friend constexpr weak_ordering operator<=>(unspecified, weak_ordering v) noexcept;
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+ };
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+
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+ // valid values' definitions
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+ inline constexpr weak_ordering weak_ordering::less(ord::less);
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+ inline constexpr weak_ordering weak_ordering::equivalent(eq::equivalent);
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+ inline constexpr weak_ordering weak_ordering::greater(ord::greater);
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+ }
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+ ```
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+
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+ ``` cpp
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+ constexpr operator partial_ordering() const noexcept;
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+ ```
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+
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+ *Returns:*
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+
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+ ``` cpp
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+ value == 0 ? partial_ordering::equivalent :
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+ value < 0 ? partial_ordering::less :
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+ partial_ordering::greater
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+ ```
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+
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+ ``` cpp
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+ constexpr bool operator==(weak_ordering v, unspecified) noexcept;
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+ constexpr bool operator< (weak_ordering v, unspecified) noexcept;
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+ constexpr bool operator> (weak_ordering v, unspecified) noexcept;
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+ constexpr bool operator<=(weak_ordering v, unspecified) noexcept;
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+ constexpr bool operator>=(weak_ordering v, unspecified) noexcept;
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+ ```
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+
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+ *Returns:* `v.value 0` for `operator`.
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+
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+ ``` cpp
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+ constexpr bool operator< (unspecified, weak_ordering v) noexcept;
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+ constexpr bool operator> (unspecified, weak_ordering v) noexcept;
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+ constexpr bool operator<=(unspecified, weak_ordering v) noexcept;
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+ constexpr bool operator>=(unspecified, weak_ordering v) noexcept;
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+ ```
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+
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+ *Returns:* `0 v.value` for `operator`.
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+
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+ ``` cpp
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+ constexpr weak_ordering operator<=>(weak_ordering v, unspecified) noexcept;
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+ ```
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+
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+ *Returns:* `v`.
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+
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+ ``` cpp
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+ constexpr weak_ordering operator<=>(unspecified, weak_ordering v) noexcept;
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+ ```
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+
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+ *Returns:*
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+ `v < 0 ? weak_ordering::greater : v > 0 ? weak_ordering::less : v`.
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+