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std::ranges::max_element

Defined in header <algorithm>

Call signature

Defined in header `<algorithm>`
Call signature
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 max_element( I first, S last, Comp comp = {}, Proj proj = {} );	(1)	(since C++20)
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> max_element( R&& r, Comp comp = {}, Proj proj = {} );	(2)	(since C++20)

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 max_element( I first, S last, Comp comp = {}, Proj proj = {} );

(1)

(since C++20)

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>
    max_element( R&& r, Comp comp = {}, Proj proj = {} );

(2)

(since C++20)

1) Finds the greatest element in the range [first, last).

2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	iterator-sentinel pair denoting the range to examine
r	-	the range to examine
comp	-	comparison to apply to the projected elements
proj	-	projection to apply to the elements

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 first if the range is empty.

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;
        ++first;
        for (; first != last; ++first)
            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>
#include <vector>
 
int main()
{
    std::vector<int> v {3, 1, -14, 1, 5, 9};
 
    namespace ranges = std::ranges;
    auto result = ranges::max_element(v.begin(), v.end());
    std::cout << "max element at: " << 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 << "max element (absolute) at: " << ranges::distance(v.begin(), result) << '\n';
}

Output:

max element at: 5
max element (absolute) at: 2