std::partial_ordering(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💅 std::partial_ordering(3) — a deep dive from your friendly neighborhood C++ bestie

okay so this page lives in the C++ Standard Library manual (yes, the (3) is giving "section 3, library functions" — iykyk) and it's literally all about std::partial_ordering. buckle up bestie, we're comparing things that maybe don't even compare. 🦋

✨ NAME ✨

std::partial_ordering — that's it, that's the name. iconic of it honestly. 👑

🌸 Synopsis 🌸

Defined in header <compare>
class partial_ordering;      (since C++20)

So std::partial_ordering is the class type you get back as the result type of a three-way comparison that:

  • Admits all six relational operators (==, !=, <, <=, >, >=).
  • Does not imply substitutability: if a is equivalent to b, then f(a) may not be equivalent to f(b), where f denotes a function that reads only comparison-salient state that is accessible via the argument's public const members. In other words, equivalent values may be distinguishable. (yes bestie, "equivalent but not identical" is a whole vibe 💀)
  • Admits incomparable values: a < b, a == b, and a > b may all be false. the way that everything can just be no 😭

🎀 Constants 🎀

The type std::partial_ordering has four valid values, implemented as const static data members of its type:

  • less (inline constexpr) [static] — a valid value of the type std::partial_ordering indicating less-than (ordered before) relationship (public static member constant)
  • equivalent (inline constexpr) [static] — a valid value of the type std::partial_ordering indicating equivalence (neither ordered before nor ordered after) (public static member constant)
  • greater (inline constexpr) [static] — a valid value of the type std::partial_ordering indicating greater-than (ordered after) relationship (public static member constant)
  • unordered (inline constexpr) [static] — a valid value of the type std::partial_ordering indicating relationship with an incomparable value (public static member constant)

four constants, zero skips. slay. ✨

🦋 Conversions 🦋

std::partial_ordering cannot be implicitly converted to other comparison category types, while both std::strong_ordering and std::weak_ordering are implicitly-convertible to partial_ordering.

so like… they can come to our party, but we're not going to theirs. boundaries. 💅

🔥 Comparisons 🔥

Comparison operators are defined between values of this type and literal 0. This supports the expressions a <=> b == 0 or a <=> b < 0 that can be used to convert the result of a three-way comparison operator to a boolean relationship; see std::is_eq, std::is_lt, etc.

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::partial_ordering is an associated class of the arguments.

⚠️ big warning bestie: the behavior of a program that attempts to compare a partial_ordering with anything other than the integer literal 0 is undefined. not "returns something weird". undefined. periodt. 💀

The whole squad:

  • operator==
  • operator<
  • operator>
  • operator<= — compares with zero or a partial_ordering (function)
  • operator>=
  • operator<=>

🌈 operator== 🌈

friend constexpr bool operator==( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr bool operator==( partial_ordering v, partial_ordering w ) noexcept = default;   (2)

Parameters

  • v, w — std::partial_ordering values to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. true if v is equivalent, false if v is less, greater, or unordered
  2. true if both parameters hold the same value, false otherwise

⭐ operator< ⭐

friend constexpr bool operator<( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr bool operator<( /*unspecified*/ u, partial_ordering v ) noexcept;   (2)

Parameters

  • v — a std::partial_ordering value to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. true if v is less, and false if v is greater, equivalent, or unordered
  2. true if v is greater, and false if v is less, equivalent, or unordered

💜 operator<= 💜

friend constexpr bool operator<=( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr bool operator<=( /*unspecified*/ u, partial_ordering v ) noexcept;   (2)

Parameters

  • v — a std::partial_ordering value to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. true if v is less or equivalent, and false if v is greater or unordered
  2. true if v is greater or equivalent, and false if v is less or unordered

🍓 operator> 🍓

friend constexpr bool operator>( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr bool operator>( /*unspecified*/ u, partial_ordering v ) noexcept;   (2)

Parameters

  • v — a std::partial_ordering value to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. true if v is greater, and false if v is less, equivalent, or unordered
  2. true if v is less, and false if v is greater, equivalent, or unordered

🧚 operator>= 🧚

friend constexpr bool operator>=( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr bool operator>=( /*unspecified*/ u, partial_ordering v ) noexcept;   (2)

Parameters

  • v — a std::partial_ordering value to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. true if v is greater or equivalent, and false if v is less or unordered
  2. true if v is less or equivalent, and false if v is greater or unordered

👑 operator<=> 👑

friend constexpr partial_ordering operator<=>( partial_ordering v, /*unspecified*/ u ) noexcept;   (1)
friend constexpr partial_ordering operator<=>( /*unspecified*/ u, partial_ordering v ) noexcept;   (2)

Parameters

  • v — a std::partial_ordering value to check
  • u — an unused parameter of any type that accepts literal zero argument

Return value

  1. v.
  2. greater if v is less, less if v is greater, otherwise v.

(yeah, return value #1 is literally just "v." — nothing more, nothing less. she said "I am the answer" 💅)

💖 Notes 💖

The built-in operator<=> between floating-point values uses this ordering: the positive zero and the negative zero compare equivalent, but can be distinguished, and NaN values compare unordered with any other value.

so 0.0 == -0.0 is true but they're not the same and NaN refuses to be compared at all. main character energy, honestly. 😭

🌸 Example 🌸

🚧 This section is incomplete Reason: no example

(ngl, they really said "figure it out bestie" on this one 🙈)

🫶 See also 🫶

  • strong_ordering (C++20) — the result type of 3-way comparison that supports all 6 operators and is substitutable (class)
  • weak_ordering (C++20) — the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)

🎀 Category 🎀

  • Todo no example

(yeah, same todo, still no example. it's giving unfinished homework. 💀)

okay that's the whole page bestie!! ✨ this one came from cppreference.com, dated 2024.06.10 — go drink some water and go compare some floats responsibly 🫶💖🌈

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