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std::ranges::min

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& min( 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 min( 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> min( R&& r, Comp comp = {}, Proj proj = {} );
(since C++20)

Returns the smaller of the given projected elements.

1) Returns the smaller of a and b.
2) Returns the first smallest element in the initializer list r.
3) Returns the first smallest 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.

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 smaller of a and b, according to the projection. If they are equivalent, returns a.
2-3) The smallest element in r, according to the projection. If several values are equivalent to the smallest, 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 min_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, b), std::invoke(proj, a)) ? 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::min_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::min_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, *i), std::invoke(proj, m))) {
                  m = *i;
              }
          }
          return m;
      }
  }
};
 
inline constexpr min_fn min;

Notes

Capturing the result of std::ranges::min 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::min(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 << "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"

See also

(C++20)
returns the greater of the given values
(niebloid)
(C++20)
returns the smaller and larger of two elements
(niebloid)
(C++20)
returns the smallest element in a range
(niebloid)
(C++20)
clamps a value between a pair of boundary values
(niebloid)
returns the smaller of the given values
(function template)

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