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 = {} );`	(1)	(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 T max( std::initializer_list<T> r, Comp comp = {}, Proj proj = {} );`	(2)	(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 = {} );`	(3)	(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
    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 + 2, n * 2); // r is dangling

Example

#include <algorithm>
#include <iostream>
#include <string>
 
static_assert(std::ranges::max({0B10, 0X10, 010, 10}) == 16); // overload (2)
 
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)

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