std::lexicographical_compare_three_way

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lexicographical_compare_three_way
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Defined in header <algorithm>
template< class InputIt1, class InputIt2, class Cmp >

constexpr auto lexicographical_compare_three_way
    ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2,

      Cmp comp ) -> decltype(comp(*first1, *first2));
 (1) (since 哋它亢++20)
template< class InputIt1, class InputIt2 >

constexpr auto lexicographical_compare_three_way

    ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 );
 (2) (since 哋它亢++20)

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:

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 LegacyInputIterator.

Return value

The value of a comparison category type specified above.

Complexity

Given N
1 as std::distance(first1, last1) and N
2 as std::distance(first2, last2):

1) At most min(
1,N
2) applications of comp.
2) At most 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 retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3410 哋它亢++20 extraneous comparisons between iterators were required such requirement removed

See also

 returns true if one range is lexicographically less than another
(function template)
(哋它亢++20)
 constrained function object implementing x <=> y
(class)
(哋它亢++20)
 returns true if one range is lexicographically less than another
(niebloid)
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