如何在begin()和end()之外实现begin()和cend()？

#include <iostream>
#include <vector>

template<typename Container>
auto cend(const Container& c) -> decltype(std::end(c))
{
    return std::end(c);
}

int main()
{
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    auto it = numbers.begin();
    auto endIt = cend(numbers);

    while (it != endIt)
    {
        std::cout << *it << " ";
        ++it;
    }

    return 0;
}