std::ranges::subrange

template<
std::input_or_output_iterator I,
std::sentinel_for<I> S = I,
ranges::subrange_kind K = std::sized_sentinel_for<S, I> ?
ranges::subrange_kind::sized :
ranges::subrange_kind::unsized
>
requires (K == ranges::subrange_kind::sized ||
!std::sized_sentinel_for<S, I>)
class subrange
: public ranges::view_interface<subrange<I, S, K>>

Helper concepts

template<class From, class To>
concept /*uses-nonqualification-pointer-conversion*/ =
/* see description */;

template<class From, class To>
concept /*convertible-to-non-slicing*/ = /* see description */;

#include <map>
#include <print>
#include <ranges>
 
void make_uppercase(char& v)
{
    v += 'A' - 'a';
}
 
void uppercase_transform(std::multimap<int, char>& m, int k)
{
    auto [first, last] = m.equal_range(k);
    for (auto& [_, v] : std::ranges::subrange(first, last))
        make_uppercase(v);
}
 
int main()
{
    std::multimap<int, char> mm{{4, 'a'}, {3, '-'}, {4, 'b'}, {5, '-'}, {4, 'c'}};
    std::println("Before: {}", mm);
    uppercase_transform(mm, 4);
    std::println("After:  {}", mm);
}