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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: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
ranges::max
Permutations
 
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< class 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:

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

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<class 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
  {
      return *ranges::max_element(r, std::ref(comp), std::ref(proj));
  }
};
 
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-1, n+1);
// r is dangling

[edit] 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"

[edit] See also

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