std::lexicographical_compare_three_way

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 C++20)
template< class InputIt1, class InputIt2 > constexpr auto lexicographical_compare_three_way( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 );	(2)	(since C++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());

Parameters

first1, last1	-	the first range of elements to examine
first2, last2	-	the second range of elements to examine
comp	-	a function object type. The program is ill-formed if its return type is not one of the three comparison category types (`std::strong_ordering`, `std::weak_ordering`, or `std::partial_ordering`).
Type requirements
- `InputIt1, InputIt2` must meet the requirements of LegacyInputIterator.

Return value

The value of a comparison category type specified above.

Complexity

At most N applications of comp, where N is the smaller of length of both ranges.

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 C++ standards.

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

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Feature test macros (C++20)
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Technical specifications
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External libraries

lexicographical_compare	returns true if one range is lexicographically less than another (function template)
compare_three_way (C++20)	function object implementing x <=> y (class)
ranges::lexicographical_compare (C++20)	returns true if one range is lexicographically less than another (niebloid)