std::ranges::max() algorithm

od C++20

Simplified

// (1)
constexpr const T&
	max( const T& a, const T& b, Comp comp = {}, Proj proj = {} );

// (2)
constexpr const T
    max( std::initializer_list<T> r, Comp comp = {}, Proj proj = {} );

// (3)
constexpr ranges::range_value_t<R> max( R&& r, Comp comp = {}, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• T - (none)
• Proj - (none)
• Comp - (none)

The Proj and Comp template arguments have, the following default types for all overloads: std::identity, ranges::less.

Additionally, each overload has the following constraints:

• (1) - indirect_strict_weak_order<Comp, projected<const T*, Proj>>
• (2) - indirect_strict_weak_order<Comp, projected<const T*, Proj>>
• (3) - indirectly_copyable_storable<ranges::iterator_t<R>, ranges::range_value_t<R>*> && indirect_strict_weak_order<Comp, projected<ranges::iterator_t<R>, Proj>>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
template<
	class T,
	class Proj = std::identity,
	class Comp = ranges::less
>
	requires std::indirect_strict_weak_order<Comp, std::projected<const T*, Proj>>
constexpr const T&
	max( const T& a, const T& b, Comp comp = {}, Proj proj = {} );

// (2)
template<
	std::copyable T,
	class Proj = std::identity,
	class Comp = ranges::less
>
	requires std::indirect_strict_weak_order<Comp, std::projected<const T*, Proj>>
constexpr const T
    max( std::initializer_list<T> r, Comp comp = {}, Proj proj = {} );

// (3)
template<
	ranges::input_range R,
	class Proj = std::identity,
	class Comp = ranges::less
>
	requires
		std::indirectly_copyable_storable<ranges::iterator_t<R>, ranges::range_value_t<R>*>
		&& std::indirect_strict_weak_order<Comp, std::projected<ranges::iterator_t<R>, Proj>>
constexpr ranges::range_value_t<R> max( R&& r, Comp comp = {}, Proj proj = {} );

Returns the greater of the given projected elements.

• (1) Returns the greatest of a and b.

• (2) Returns the first greatest element 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.

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 the projection. If they are equivalent, returns a.
• (3 - 4) The greatest element in r, according to the projection. If several values are equivalent to the smallest, returns the leftmost one.

Undefined Behavior

If the range is empty (as determined by ranges::distance(r)), the behavior is undefined.

Complexity

• (1) Exactly one comparison.
• (3 - 4) Exactly ranges::distance(r) - 1 comparisons.

Exceptions

(none)

Possible implementation

max(1) and max(2)

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

Undefined Behavior

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

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <string>

int main()
{
    namespace ranges = std::ranges;
    using namespace std::string_view_literals;

    std::cout << "smaller of 1 and 9999: " << ranges::min(1, 9999) << '\n'
              << "smaller of 'a', and 'b': '" << ranges::min('a', 'b') << "'\n"
              << "shortest of \"foo\", \"bar\", and \"hello\": \""
              << ranges::min({ "foo"sv, "bar"sv, "hello"sv }, {},
                             &std::string_view::size) << "\"\n";
}

Output

smaller of 1 and 9999: 1
smaller of 'a', and 'b': 'a'
shortest of "foo", "bar", and "hello": "foo"