Template class not inheriting the protected variable from another Template class - c++

Question

I'm making a Linked list which is generic in nature and have some basic functionality. Then I'm trying to make another template class called "Set" which is inheriting from LinkedList. But when I try to access "head" which is a Node<>* defined in Linked List. It's giving an error. My files are:
LinkedList.h
template <typename T>
struct Node {
T data;
Node<T> *next;
};
template <typename T>
class LinkedList {
public:
Node<T>* head;
int size;
LinkedList();
LinkedList(const LinkedList<T> &lst);
~LinkedList();
Node<T>* getHead();
Node<T>* getTail();
};
template <typename T>
class Set:public LinkedList<T> {
public:
void insert(T item);
friend ostream&(ostream& out, const Set<T> set)
};
and an implementation of insert is:
template <typename T>
void Set<T>::insert(T item) {
Node<T>* temp = head;
bool present = false;
while (temp != NULL) {
if (temp->data == item) {
present = true;
}
temp = temp->next;
}
if (present == false) {
/*Node<T> *tail = getTail();
Node<T>* newTail = new Node<T>(item);
newTail->next = NULL;
tail->next = newTail;*/
}
}
It says:
error: "head" was not declared in this scope in line "Node<T>* temp = head"

Answer 1

This C++ oddity is due to two-phase lookup and the fact that head is a dependant name (as a member of a base class that depends on the "current" class's template arguments):
[C++11: 14.6.2/3]: In the definition of a class or class template, if a base class depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member. [..]
Bypass unqualified lookup by introducing this into the expression (per [C++11: 3.4.5]):
Node<T>* temp = this->head;
// ^^^^^^
There's a longer explanation on this previous Stack Overflow answer:
Why do I have to access template base class members through the this pointer?
Here's a minimal(ish) testcase:
#include <iostream>
template <typename T>
struct Base
{
int x = 42;
};
template <typename T>
struct Derived : Base<T>
{
void foo();
};
template <typename T>
void Derived<T>::foo()
{
std::cout << x << '\n';
}
int main()
{
Derived<void> d;
d.foo();
}
// main.cpp: In member function 'void Derived<T>::foo()':
// main.cpp:18:18: error: 'x' was not declared in this scope
// std::cout << x << '\n';
// ^
(live demo)
To fix, change foo thus:
template <typename T>
void Derived<T>::foo()
{
std::cout << this->x << '\n';
}
(live demo)

Answer 2

You're inheriting from a dependent base class so member access needs to be qualified with this:
Node<T>* temp = this->head;
For more information, see this thread.