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std::ranges::max

Defined in header `<algorithm>`
Call signature
template< class T, class Proj = std::identity, std::indirect_strict_weak_order< std::projected<const T*, Proj>> Comp = ranges::less > constexpr const T& max( const T& a, const T& b, Comp comp = {}, Proj proj = {} );		(since C++20)
template< std::copyable T, class Proj = std::identity, std::indirect_strict_weak_order< std::projected<const T*, Proj>> Comp = ranges::less > constexpr const T max( std::initializer_list<T> r, Comp comp = {}, Proj proj = {} );		(since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirect_strict_weak_order< std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > requires std::indirectly_copyable_storable<ranges::iterator_t<R>, ranges::range_value_t<R>*> constexpr ranges::range_value_t<R> max( R&& r, Comp comp = {}, Proj proj = {} );		(since C++20)

Returns the greater of the given projected values.

1) Returns the greater of a and b.

2) Returns the first greatest value in the initializer list r.

3) Returns the first greatest value in the range r.

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

a, b	-	the values to compare
r	-	the range of values to compare
comp	-	comparison to apply to the projected elements
proj	-	projection to apply to the elements

Return value

1) The greater of a and b, according to their respective projected values. If they are equivalent, returns a.

2-3) The greatest value in r, according to the projection. If several values are equivalent to the greatest, returns the leftmost one. If the range is empty (as determined by ranges::distance(r)), the behavior is undefined.

Complexity

1) Exactly one comparison.

2-3) Exactly ranges::distance(r) - 1 comparisons.

Possible implementation

struct max_fn
{
    template<class T, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<const T*, Proj>> Comp = ranges::less>
    constexpr
    const T& operator()(const T& a, const T& b, Comp comp = {}, Proj proj = {}) const
    {
        return std::invoke(comp, std::invoke(proj, a), std::invoke(proj, b)) ? b : a;
    }
 
    template<std::copyable T, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<const T*, Proj>> Comp = ranges::less>
    constexpr
    const T operator()(std::initializer_list<T> r, Comp comp = {}, Proj proj = {}) const
    {
        return *ranges::max_element(r, std::ref(comp), std::ref(proj));
    }
 
    template<ranges::input_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                  std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    requires std::indirectly_copyable_storable<ranges::iterator_t<R>,
                                               ranges::range_value_t<R>*>
    constexpr
    ranges::range_value_t<R> operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        using V = ranges::range_value_t<R>;
        if constexpr (ranges::forward_range<R>)
            return
                static_cast<V>(*ranges::max_element(r, std::ref(comp), std::ref(proj)));
        else
        {
            auto i = ranges::begin(r);
            auto s = ranges::end(r);
            V m(*i);
            while (++i != s)
                if (std::invoke(comp, std::invoke(proj, m), std::invoke(proj, *i)))
                    m = *i;
            return m;
        }
    }
};
 
inline constexpr max_fn max;

Notes

Capturing the result of std::ranges::max by reference produces a dangling reference if one of the parameters is a temporary and that parameter is returned:

int n = 1;
const int& r = std::ranges::max(n - 1, n + 1); // r is dangling

Example

#include <algorithm>
#include <iostream>
#include <string>
 
int main()
{
    namespace ranges = std::ranges;
    using namespace std::string_view_literals;
 
    std::cout << "larger of 1 and 9999: " << ranges::max(1, 9999) << '\n'
              << "larger of 'a', and 'b': '" << ranges::max('a', 'b') << "'\n"
              << "longest of \"foo\", \"bar\", and \"hello\": \""
              << ranges::max({ "foo"sv, "bar"sv, "hello"sv }, {},
                             &std::string_view::size) << "\"\n";
}

Output:

larger of 1 and 9999: 9999
larger of 'a', and 'b': 'b'
longest of "foo", "bar", and "hello": "hello"

ranges::min (C++20)	returns the smaller of the given values (niebloid)
ranges::minmax (C++20)	returns the smaller and larger of two elements (niebloid)
ranges::max_element (C++20)	returns the largest element in a range (niebloid)
ranges::clamp (C++20)	clamps a value between a pair of boundary values (niebloid)
max	returns the greater of the given values (function template)