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#include <iostream>
#include <string>
#include <initializer_list>
 
template <class T>
class LinkedList
{
private:
	struct node
	{
		T data;
		node *next;
	} *head = nullptr;
public:
	// Constructors and destructors
	LinkedList() = default;
	~LinkedList();
	LinkedList(const std::initializer_list<T> &il);
	LinkedList(std::istream &is);
 
	// Member functions
	void push (T data);
	void reverse();
	friend std::ostream& operator << (std::ostream &os, const LinkedList<T> &ls);
};
 
template <class T>
LinkedList<T>::~LinkedList()
{
	node *cur = head, *next = nullptr;
	while (cur)
	{
		next = cur->next;
		delete cur;
		cur = next;
	}
}
 
template <class T>
LinkedList<T>::LinkedList(const std::initializer_list<T> &il)
{
	for (const auto &data : il)
		push (data);
}
 
template <class T>
LinkedList<T>::LinkedList(std::istream &is)
{
	T temp;
	while (is >> temp)
		push (temp);
}
 
// Push to head
template <class T>
void LinkedList<T>::push (T data)
{
	node *elem = new node;
	elem->data = data;
	elem->next = head;
	head = elem;
}
 
template <class T>
void LinkedList<T>::reverse()
{
	node *prev = nullptr;
	node *next = nullptr;
	if (!head) return;
	while (head->next)
	{
		next = head->next;
		head->next = prev;
		prev = head;
		head = next;
	}
	head->next = prev;
}
 
template <class T>
std::ostream& operator << (std::ostream &os, const LinkedList<T> &ls)
{
	LinkedList<T>::node *iter = ls.head;
	while (iter)
	{
		os << iter->data << " ";
		iter = iter->next;
	}
	return os;
}
 
 
int main()
{
	// Integers
	LinkedList<int> list{1, 2, 3, 4, 5};
	std::cout << list << std::endl;
	list.reverse();
	std::cout << list << std::endl;
 
	// Strings
	LinkedList<std::string> s(std::cin);
	std::cout << s << std::endl;
	s.reverse();
	std::cout << s << std::endl;
}

prog.cpp:24:77: warning: friend declaration 'std::ostream& operator<<(std::ostream&, const LinkedList<T>&)' declares a non-template function [-Wnon-template-friend]
  friend std::ostream& operator << (std::ostream &os, const LinkedList<T> &ls);
                                                                             ^
prog.cpp:24:77: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) 
prog.cpp: In function 'std::ostream& operator<<(std::ostream&, const LinkedList<T>&)':
prog.cpp:83:23: error: 'iter' was not declared in this scope
  LinkedList<T>::node *iter = ls.head;
                       ^

Standard output is empty