std::ranges::min - cppreference.com

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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 = {} );	(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 min( 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> min( R&& r, Comp comp = {}, Proj proj = {} );	(3)	(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 algorithm function objects (informally known as 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.

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
    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 + 2, n * 2); // 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"

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