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std::lexico...e_three_way(3) C++ Standard Libary std::lexico...e_three_way(3) NAME std::lexicographical_compare_three_way - std::lexicographical_com- pare_three_way Synopsis Defined in header <algorithm> template< class InputIt1, class InputIt2, class Cmp > constexpr auto lexicographical_compare_three_way ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 (1) (since C++20) last2, Cmp comp ) -> decltype(comp(*first1, *first2)); template< class InputIt1, class InputIt2 > constexpr auto lexicographical_compare_three_way (2) (since C++20) ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); Lexicographically compares two ranges [first1, last1) and [first2, last2) using three-way comparison and produces a result of the strongest applica- ble comparison category type. 1) Returns the order between the first non-equivalent pair of ele- ments according to comp in both ranges if any, otherwise (if one ranges is equivalent to the prefix of another according to comp), returns the order between the length of both ranges. 2) Equivalent to return std::lexicographical_compare_three_way( first1, last1, first2, last2, std::compare_three_way()); If the return type is not one of the three comparison category types, the program is ill-formed: * std::strong_ordering * std::weak_ordering * std::partial_ordering Parameters first1, last1 - the first range of elements to examine first2, last2 - the second range of elements to examine comp - a function object Type requirements - InputIt1, InputIt2 must meet the requirements of LegacyInputItera- tor. Return value The value of a comparison category type specified above. Complexity Given \(\scriptsize N_1\)N 1 as std::distance(first1, last1) and \(\scriptsize N_2\)N 2 as std::distance(first2, last2): 1) At most \(\scriptsize \min(N_1,N_2)\)min( 1,N 2) applications of comp. 2) At most \(\scriptsize \min(N_1,N_2)\)min(N 1,N 2) applications of std::compare_three_way(). Possible implementation template<class I1, class I2, class Cmp> constexpr auto lexicographical_compare_three_way(I1 f1, I1 l1, I2 f2, I2 l2, Cmp comp) -> decltype(comp(*f1, *f2)) { using ret_t = decltype(comp(*f1, *f2)); static_assert(std::disjunction_v< std::is_same<ret_t, std::strong_ordering>, std::is_same<ret_t, std::weak_ordering>, std::is_same<ret_t, std::partial_ordering>>, "The return type must be a comparison category type."); bool exhaust1 = (f1 == l1); bool exhaust2 = (f2 == l2); for (; !exhaust1 && !exhaust2; exhaust1 = (++f1 == l1), exhaust2 = (++f2 == l2)) if (auto c = comp(*f1, *f2); c != 0) return c; return !exhaust1 ? std::strong_ordering::greater: !exhaust2 ? std::strong_ordering::less: std::strong_ordering::equal; } Example // Run this code #include <algorithm> #include <cctype> #include <compare> #include <iomanip> #include <iostream> #include <string_view> #include <utility> using namespace std::literals; void show_result(std::string_view s1, std::string_view s2, std::strong_ordering o) { std::cout << std::quoted(s1) << " is "; std::is_lt(o) ? std::cout << "less than ": std::is_gt(o) ? std::cout << "greater than ": std::cout << "equal to "; std::cout << std::quoted(s2) << '\n'; } std::strong_ordering cmp_icase(unsigned char x, unsigned char y) { return std::toupper(x) <=> std::toupper(y); }; int main() { for (const auto& [s1, s2] : { std::pair{"one"sv, "ONE"sv}, {"two"sv, "four"sv}, {"three"sv, "two"sv} }) { const auto res = std::lexicographical_compare_three_way( s1.cbegin(), s1.cend(), s2.cbegin(), s2.cend(), cmp_icase); show_result(s1, s2, res); } } Output: "one" is equal to "ONE" "two" is greater than "four" "three" is less than "two" 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 3410 C++20 extraneous comparisons between such re- quirement removed iterators were required See also returns true if one range is lexico- graphically less lexicographical_compare than another (function template) compare_three_way constrained function object imple- menting x <=> y (C++20) (class) ranges::lexicographical_compare returns true if one range is lexico- graphically less (C++20) than another (niebloid) http://cppreference.com 2024.06.10 std::lexico...e_three_way(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Possible implementation | Example | Output: | See also
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