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std::compare_three_way(3) C++ Standard Libary std::compare_three_way(3) NAME std::compare_three_way - std::compare_three_way Synopsis Defined in header <compare> Defined in header <functional> struct compare_three_way; (since C++20) Function object for performing comparisons. Deduces the parameter types and the return type of the function call operator. Implementation-defined strict total order over pointers The function call operator yields the implementation-defined strict total order over pointers if the <=> operator between arguments invokes a built-in comparison operator for a pointer, even if the built-in <=> operator does not. The implementation-defined strict total order is consistent with the partial order imposed by built-in comparison operators (<=>, <, >, <=, and >=), and consistent among following standard function objects: * std::less, std::greater, std::less_equal, and std::greater_equal, when the template argument is a pointer type or void * std::ranges::equal_to, std::ranges::not_equal_to, std::ranges::less, std::ranges::greater, std::ranges::less_equal, std::ranges::greater_equal, and std::compare_three_way Member types Member type Definition is_transparent /* unspecified */ Member functions operator() obtains the result of three-way comparison on both argu- ments (public member function) std::compare_three_way::operator() template< class T, class U > requires std::three_way_comparable_with<T, U> // with different se- mantic requirements constexpr auto operator()( T&& t, U&& u ) const; Compares t and u, equivalent to return std::forward<T>(t) <=> std::forward<U>(u);, except when that expression resolves to a call to a builtin opera- tor<=> comparing pointers. When a call would not invoke a built-in operator comparing pointers, the behavior is undefined if std::three_way_comparable_with<T, U> is not modeled. When a call would invoke a built-in operator comparing pointers of type P, the result is instead determined as follows: * Returns std::strong_ordering::less if the (possibly converted) value of the first argument precedes the (possibly converted) value of the second argument in the implementation-defined strict total ordering over all pointer values of type P. This strict total ordering is consistent with the partial or- der imposed by the builtin operators <, >, <=, and >=. * Otherwise, returns std::strong_ordering::greater if (possibly converted) value of the second argument precedes the (possibly converted) value of the first argument in the same strict total ordering. * Otherwise, returns std::strong_ordering::equal. The behavior is undefined unless the conversion sequences from both T and U to P are equality-preserving (see below). Equality preservation An expression is equality preserving if it results in equal outputs given equal inputs. * The inputs to an expression consist of its operands. * The outputs of an expression consist of its result and all operands modified by the expression (if any). In specification of standard concepts, operands are defined as the largest subexpressions that include only: * an id-expression, and * invocations of std::move, std::forward, and std::declval. The cv-qualification and value category of each operand is deter- mined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type. Every expression required to be equality preserving is further re- quired to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects. Example // Run this code #include <iostream> #include <compare> struct Rational { int num; int den; // > 0 // Although the comparison X <=> Y will work, a direct call // to std::compare_three_way{}(X,Y) requires the operator== // be defined, to satisfy the std::three_way_comparable_with. constexpr bool operator==(Rational const&) const = default; }; constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs) { return lhs.num * rhs.den <=> rhs.num * lhs.den; } void print(std::weak_ordering value) { value < 0 ? std::cout << "less\n" : value > 0 ? std::cout << "greater\n" : std::cout << "equal\n" ; } int main() { Rational a{6,5}; Rational b{8,7}; print(a <=> b); print(std::compare_three_way{}(a,b)); } Output: greater greater Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 3530 C++20 syntactic checks were relaxed while only se- mantic requirements comparing pointers relaxed See also ranges::equal_to function object implementing x == y (C++20) (class) ranges::not_equal_to function object implementing x != y (C++20) (class) ranges::less function object implementing x < y (C++20) (class) ranges::greater function object implementing x > y (C++20) (class) ranges::less_equal function object implementing x <= y (C++20) (class) ranges::greater_equal function object implementing x >= y (C++20) (class) http://cppreference.com 2022.07.31 std::compare_three_way(3)
NAME | Synopsis | Member types | Member functions | Example | Output: | See also
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