std::flat_multimap<Key,T,Compare,KeyContainer,MappedContainer>:: keys

#include <flat_map>
#include <print>
#include <type_traits>
#include <vector>
int main()
{
    std::flat_multimap<int, double> adaptor{{1, 1.1}, {2, 2.2}, {3, 3.3}};
    // Der Standard-Key-Container ist std::vector:
    static_assert(std::is_same_v<decltype(adaptor.keys()), const std::vector<int>&>);
    std::println("{}", adaptor.keys());
}