#include <iostream>
#include <cassert>
#include <memory>

template <typename T>
class Node
{
    std::shared_ptr<Node<T>> nextnode;
    T value;

public:
    Node() {}
    Node(T v) : value(v), nextnode(nullptr) {}
    Node(T v, std::shared_ptr<Node<T>> n) : value(v), nextnode(n) {}

    auto get() const noexcept -> decltype(value)
    {
        return value;
    }

    auto next() const noexcept -> decltype(nextnode)
    {
        return nextnode;
    }

};

template <typename T>
class Stack
{
    std::shared_ptr<Node<T>> head;

public:
    Stack() : head(nullptr) {}

    void push(T v)
    {
        auto aux = std::make_shared<Node<T>>(Node<T>(v, head));
        head = aux;
    }

    auto pop() -> decltype(head.get()->get())
    {
        if(empty()) return 0;
        auto aux = head;
        head = head->next();
        return aux.get()->get();
    }

    bool empty()
    {
        return head == nullptr;
    }

    template <typename F>
    void apply(F f)
    {
        for(auto aux = head; aux; aux = aux->next())
            f(aux.get()->get());
    }

};

int main() {
    Stack<int> s;

    s.push(10);
    s.push(15);

    s.apply([](int s) { std::cout << s << std::endl; });

    assert(s.pop() == 15);
    assert(s.pop() == 10);
}