std::ranges:: max_element

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup .
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

Return value

Iterator to the greatest element in the range [ first , last ) . If several elements in the range are equivalent to the greatest element, returns the iterator to the first such element. Returns the iterator that compares equal to last if the range is empty (i.e. if first == last ).

Complexity

Exactly max(N - 1, 0) comparisons, where N = ranges:: distance ( first, last ) .

Possible implementation

struct max_element_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == last)
            return last;
        auto largest = first;
        while (++first != last)
            if (std::invoke(comp, std::invoke(proj, *largest), std::invoke(proj, *first)))
                largest = first;
        return largest;
    }
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
    }
};
inline constexpr max_element_fn max_element;

Example

#include <algorithm>
#include <cmath>
#include <iostream>
int main()
{
    namespace ranges = std::ranges;
    const auto v = {3, 1, -14, 1, 5, 9, -14, 9};
    auto result = ranges::max_element(v.begin(), v.end());
    std::cout << "Max element at pos " << ranges::distance(v.begin(), result) << '\n';
    auto abs_compare = [](int a, int b) { return std::abs(a) < std::abs(b); };
    result = ranges::max_element(v, abs_compare);
    std::cout << "Absolute max element at pos "
              << ranges::distance(v.begin(), result) << '\n';
}

Max element at pos 5
Absolute max element at pos 2

See also