std::lexicographical_compare_three_way - cppreference.com (original) (raw)

Lexicographically compares two ranges [first1, last1) and [first2, last2) using three-way comparison and produces a result of the strongest applicable comparison category type.

1. Returns the order between the first non-equivalent pair of elements 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

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Example
• 6 Defect reports
• 7 See also

[edit] Parameters

[edit] Return value

The value of a comparison category type specified above.

[edit] Complexity

Given \(\scriptsize N_1\)N1 as std::distance(first1, last1) and \(\scriptsize N_2\)N2 as std::distance(first2, last2):

1. At most \(\scriptsize \min(N_1,N_2)\)min(1,N2) applications of comp.

2. At most \(\scriptsize \min(N_1,N_2)\)min(N1,N2) applications of std::compare_three_way().

[edit] 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; }

[edit] Example

#include #include #include #include #include #include #include   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"

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also