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_or- dering [static] indicating less-than (ordered before) rela- tionship (public static member constant) a valid value of the type std::strong_or- dering equivalent(inline constexpr) indicating equivalence (neither ordered be- fore nor [static] ordered after), the same as equal (public static member constant) a valid value of the type std::strong_or- dering equal(inline constexpr) indicating equivalence (neither ordered be- fore nor [static] ordered after), the same as equivalent (public static member constant) greater(inline constexpr) a valid value of the type std::strong_or- dering [static] indicating greater-than (ordered after) re- lationship (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-con- vertible to the other two. operator partial_ordering implicit conversion to std::partial_ordering (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_ordering::greater if v is greater, std::partial_ordering::equivalent if v is equal or equivalent. 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 (1) operator==( strong_ordering v, /*unspecified*/ u ) noexcept; friend constexpr bool (2) operator==( strong_ordering v, strong_ordering w ) noexcept = default; Parameters v, w - std::strong_ordering values to check u - an unused parameter of any type that accepts literal zero argu- ment 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 ) (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 argument 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 argument 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 ) (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 argument 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 argument 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 (1) operator<=>( strong_ordering v, /*unspecified*/ u ) noexcept; friend constexpr strong_ordering (2) operator<=>( /*unspecified*/ u, strong_ordering v ) noexcept; Parameters v - a std::strong_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. Example // Run this code #include <compare> #include <iostream> struct Point { int x{}, y{}; friend constexpr std::strong_ordering operator<=>(Point lhs, Point rhs) { if (lhs.x < rhs.x or (lhs.x == rhs.x and lhs.y < rhs.y)) return std::strong_ordering::less; if (lhs.x > rhs.x or (lhs.x == rhs.x and lhs.y > rhs.y)) return std::strong_ordering::greater; return std::strong_ordering::equivalent; } friend std::ostream& operator<<(std::ostream& os, Point s) { return os << '(' << s.x << ',' << s.y << ')'; } }; void print_three_way_comparison(const auto& p, const auto& q) { const auto cmp{p <=> q}; std::cout << p << (cmp < 0 ? " < " : cmp > 0 ? " > " : " == " ) // com- pares with 0 << q << '\n'; } void print_two_way_comparison(const auto& p, const auto& q) { std::cout << p << (p < q ? " < " : p > q ? " > " : " == ") // compares p and q << q << '\n'; } int main() { const Point p1{0, 1}, p2{0, 1}, p3{0, 2}; print_three_way_comparison(p1, p2); print_two_way_comparison(p1, p2); print_three_way_comparison(p2, p3); print_two_way_comparison(p2, p3); print_three_way_comparison(p3, p2); print_two_way_comparison(p3, p2); } Output: (0,1) == (0,1) (0,1) == (0,1) (0,1) < (0,2) (0,1) < (0,2) (0,2) > (0,1) (0,2) > (0,1) 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 values (class) http://cppreference.com 2024.06.10 std::strong_ordering(3)
NAME | Synopsis | Constants | Conversions | Return value | Return value | Comparisons | Parameters | Return value | Parameters | Return value | Parameters | Return value | Parameters | Return value | Parameters | Return value | Parameters | Return value | Example | Output: | See also
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