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std::strong_ordering(3) C++ Standard Libary std::strong_ordering(3) NAME std::strong_ordering - std::strong_ordering Synopsis Defined in header <compare> class strong_ordering; (since C++20) The class type std::strong_ordering is the result type of a three- way comparison that * admits all six relational operators (==, !=, <, <=, >, >=) * implies substitutability: if a is equivalent to b, f(a) is also 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 are indistinguishable. * does not allow incomparable values: exactly one of a < b, a == b, or a > b must be true Constants The type std::strong_ordering has four valid values, implemented as const static data members of its type: Member constant Definition less(inline constexpr) a valid value of the type std::strong_ordering [static] indicating less-than (ordered before) relationship (public static member constant) a valid value of the type std::strong_ordering equivalent(inline constexpr) indicating equivalence (neither ordered before nor [static] ordered after), the same as equal (public static member constant) a valid value of the type std::strong_ordering equal(inline constexpr) indicating equivalence (neither ordered before nor [static] ordered after), the same as equivalent (public static member constant) greater(inline constexpr) a valid value of the type std::strong_ordering [static] indicating greater-than (ordered after) relationship (public static member constant) Conversions std::strong_ordering is the strongest of the three comparison cate- gories: it is not implicitly-convertible from any other category and is implicitly- convertible to the other two. operator partial_ordering implicit conversion to std::partial_order- ing (public member function) std::strong_ordering::operator partial_ordering constexpr operator partial_ordering() const noexcept; Return value std::partial_ordering::less if v is less, std::partial_order- ing::greater if v is greater, std::partial_ordering::equivalent if v is equal or equiva- lent. operator weak_ordering implicit conversion to std::weak_ordering (public member function) std::strong_ordering::operator weak_ordering constexpr operator weak_ordering() const noexcept; Return value std::weak_ordering::less if v is less, std::weak_ordering::greater if v is greater, std::weak_ordering::equivalent if v is equal or equivalent. 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::strong_ordering is an associated class of the arguments. The behavior of a program that attempts to compare a strong_ordering with anything other than the integer literal 0 is undefined. operator== operator< operator> compares with zero or a strong_ordering operator<= (function) operator>= operator<=> operator== friend constexpr bool operator==(strong_ordering v, /*unspecified*/ u) (1) noexcept; friend constexpr bool operator==(strong_ordering v, strong_ordering w) noexcept (2) = default; Parameters v, w - std::strong_ordering values to check u - an unused parameter of any type that accepts literal zero ar- gument Return value 1) true if v is equivalent or equal, false if v is less or greater 2) true if both parameters hold the same value, false otherwise. Note that equal is the same as equivalent. operator< friend constexpr bool operator<(strong_ordering v, /*unspecified*/ u) noexcept; (1) friend constexpr bool operator<(/*unspecified*/ u, strong_ordering v) noexcept; (2) Parameters v - a std::strong_ordering value to check u - an unused parameter of any type that accepts literal zero argu- ment Return value 1) true if v is less, and false if v is greater, equivalent, or equal 2) true if v is greater, and false if v is less, equivalent, or equal operator<= friend constexpr bool operator<=(strong_ordering v, /*unspecified*/ u) (1) noexcept; friend constexpr bool operator<=(/*unspecified*/ u, strong_ordering v) (2) noexcept; Parameters v - a std::strong_ordering value to check u - an unused parameter of any type that accepts literal zero argu- ment Return value 1) true if v is less, equivalent, or equal, and false if v is greater 2) true if v is greater, equivalent, or equal, and false if v is less operator> friend constexpr bool operator>(strong_ordering v, /*unspecified*/ u) noexcept; (1) friend constexpr bool operator>(/*unspecified*/ u, strong_ordering v) noexcept; (2) Parameters v - a std::strong_ordering value to check u - an unused parameter of any type that accepts literal zero argu- ment Return value 1) true if v is greater, and false if v is less, equivalent, or equal 2) true if v is less, and false if v is greater, equivalent, or equal operator>= friend constexpr bool operator>=(strong_ordering v, /*unspecified*/ u) (1) noexcept; friend constexpr bool operator>=(/*unspecified*/ u, strong_ordering v) (2) noexcept; Parameters v - a std::strong_ordering value to check u - an unused parameter of any type that accepts literal zero argu- ment Return value 1) true if v is greater, equivalent, or equal, and false if v is less 2) true if v is less, equivalent, or equal, and false if v is greater operator<=> friend constexpr strong_ordering operator<=>(strong_ordering v, /*unspecified*/ (1) u) noexcept; friend constexpr strong_ordering operator<=>(/*unspecified*/ u, strong_ordering (2) v) noexcept; Parameters v - a std::strong_ordering value to check u - an unused parameter of any type that accepts literal zero argu- ment Return value 1) v. 2) greater if v is less, less if v is greater, otherwise v. Example This section is incomplete Reason: no example See also weak_ordering the result type of 3-way comparison that supports all 6 operators (C++20) and is not substitutable (class) partial_ordering the result type of 3-way comparison that supports all 6 operators, (C++20) is not substitutable, and allows incomparable val- ues (class) http://cppreference.com 2022.07.31 std::strong_ordering(3)
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