std::lexicographical_compare_three_way() algorithm

since C++20

// (1)
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));

// (2)
template< class InputIt1, class InputIt2 >
constexpr auto lexicographical_compare_three_way( 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 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 compare.
first2 last2	The second range of elements compare.
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

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.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

lexicographical_compare_three_way(1)

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;
}

Examples

Main.cpp

#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"