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std::ranges::max

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
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(C++17)
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(C++11)                (C++11)(C++11)

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(C++11)
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(until C++17)(C++11)
(C++20)(C++20)
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(C++17)

Sorting and related operations
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(on partitioned ranges)
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Merge operations (on sorted ranges)
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(C++11)
(C++17)
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C library
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Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
       
       
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
max
       
       
Permutation operations
Fold operations
Numeric operations
(C++23)            
Operations on uninitialized storage
Return types
 
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 algorithm function objects (informally known as niebloids), that is:

Contents

[edit] 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

[edit] 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.

[edit] Complexity

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

[edit] 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;

[edit] 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

[edit] 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"

[edit] See also

returns the smaller of the given values
(algorithm function object)[edit]
returns the smaller and larger of two elements
(algorithm function object)[edit]
returns the largest element in a range
(algorithm function object)[edit]
clamps a value between a pair of boundary values
(algorithm function object)[edit]
returns the greater of the given values
(function template) [edit]