NAME

std::lexicographical_compare_three_way - std::lexicographical_compare_three_way

Synopsis

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

constexpr auto lexicographical_compare_three_way( InputIt1 first1,
InputIt1 last1, (1) (since C++20)
InputIt2 first2, InputIt2 last2,
Cmp comp );

-> decltype(comp(*first1, *first2));
template< class InputIt1, class InputIt2 >

constexpr auto lexicographical_compare_three_way( InputIt1 first1, (2) (since C++20)
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 to examine
first2, last2 - the second range of elements to examine
a function object type. The program is ill-formed if its return type
comp - 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 << quoted(s1) << " is ";
(o < 0) ? std::cout << "less than " :
(o > 0) ? std::cout << "greater than " :
std::cout << "equal to ";
std::cout << quoted(s2) << '\n';
}

int main()
{
auto cmp_icase = [](char x, char y) {
const auto ux { std::toupper(x) };
const auto uy { std::toupper(y) };
return (ux < uy) ? std::strong_ordering::less:
(ux > uy) ? std::strong_ordering::greater:
std::strong_ordering::equal;
};

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 such requirement removed
iterators were required