C++ List Implementation - c++

Question

So, I'm building an implementation of List for a programming exercise. So far I have this:
#include <iostream>
#include <algorithm>
using namespace std;
template <class T> class Link;
template <class T> class List_iterator;
template <class T>
class List
{
public:
typedef List_iterator<T> iterator;
List();
List(const List<T> & l);
~List();
bool empty() const;
unsigned int size() const;
T & back() const;
T & front() const;
void push_front(const T & x);
void push_back(const T & x);
void pop_front();
void pop_back();
iterator begin() const;
iterator end() const;
void insert(iterator pos, const T & x);
void erase(iterator & pos);
List<T> & operator=(const List<T> & l);
protected:
Link<T> * first_link;
Link<T> * last_link;
unsigned int my_size;
};
template <class T>
List<T>::List()
{
first_link = 0;
last_link = 0;
my_size = 0;
}
template <class T>
List<T>::List(const List & l)
{
first_link = 0;
last_link = 0;
my_size = 0;
for (Link<T> * current = l.first_link; current != 0; current = current -> next_link)
push_back(current -> value);
}
template <class T>
typename List<T>::iterator List<T>::begin() const
{
return iterator(first_link);
}
template <class T>
class Link
{
private:
Link(const T & x): value(x), next_link(0), prev_link(0) {}//pg. 204
T value;
Link<T> * next_link;
Link<T> * prev_link;
friend class List<T>;
friend class List_iterator<T>;
};
template <class T> class List_iterator
{
public:
typedef List_iterator<T> iterator;
List_iterator(Link<T> * source_link): current_link(source_link) { }
List_iterator(): current_link(0) { }
List_iterator(List_iterator<T> * source_iterator): current_link(source_iterator.current_link) { }
T & operator*(); // dereferencing operator
iterator & operator=(const iterator & rhs);
bool operator==(const iterator & rhs) const;
bool operator!=(const iterator & rhs) const;
iterator & operator++();
iterator operator++(int);
iterator & operator--();
iterator operator--(int);
protected:
Link<T> * current_link;
friend class List<T>;
};
template <class T>
T & List_iterator<T>::operator*()
{
return current_link -> value;
}
template <class T>
List_iterator<T> & List_iterator<T>::operator++()
{
current_link = current_link -> next_link;
return *this;
}
template <class T>
void List<T>::push_back(const T & x)
{
link<T> * last_link = new link<T> (x);
if (empty())
first_link = last_link;
else
{
last_link->prev_link = last_link;
last_link->prev_link = last_link;
last_link = last_link;
}
}
template <class T>
typename List<T>::iterator List<T>::end() const
{
return iterator(last_link);
}
int main()
{
List<int> l;
l.push_back(44); // list = 44
l.push_back(33); // list = 44, 33
l.push_back(11); // list = 44, 33, 11
l.push_back(22); // list = 44, 33, 11, 22
List<int> m(l);
List<int>::iterator itr(m.begin());
while (itr != m.end()) {
cout << *itr << endl;
itr++;
}
}`
I'm having a lot of trouble with my push_back() function and I'm not quite sure what's wrong. Can anyone point out my errors?
Updated Code In Progress:
#include <iostream>
#include <algorithm>
using namespace std;
template <class T> class Link;
template <class T> class List_iterator;
template <class T>
class List
{
public:
typedef List_iterator<T> iterator;
List();
List(const List<T> & l);
~List();
bool empty() const;
unsigned int size() const;
T & back() const;
T & front() const;
void push_front(const T & x);
void push_back(const T & x);
void pop_front();
void pop_back();
iterator begin() const;
iterator end() const;
void insert(iterator pos, const T & x);
void erase(iterator & pos);
List<T> & operator=(const List<T> & l);
protected:
Link<T> * first_link;
Link<T> * last_link;
unsigned int my_size;
};
template <class T>
List<T>::List()
{
first_link = 0;
last_link = 0;
my_size = 0;
}
template <class T>
List<T>::List(const List & l)
{
first_link = 0;
last_link = 0;
my_size = 0;
for (Link<T> * current = l.first_link; current != 0; current = current -> next_link)
push_back(current -> value);
}
template <class T>
typename List<T>::iterator List<T>::begin() const
{
return iterator(first_link);
}
template <class T>
class Link
{
private:
Link(const T & x): value(x), next_link(0), prev_link(0) {}//pg. 204
T value;
Link<T> * next_link;
Link<T> * prev_link;
friend class List<T>;
friend class List_iterator<T>;
};
template <class T> class List_iterator//pg.207
{
public:
typedef List_iterator<T> iterator;
List_iterator(Link<T> * source_link): current_link(source_link) { }
List_iterator(): current_link(0) { }
List_iterator(List_iterator<T> * source_iterator): current_link(source_iterator.current_link) { }
T & operator*(); // dereferencing operator
iterator & operator=(const iterator & rhs);
bool operator==(const iterator & rhs) const;
bool operator!=(const iterator & rhs) const;
iterator & operator++();
iterator operator++(int);
iterator & operator--();
iterator operator--(int);
protected:
Link<T> * current_link;
friend class List<T>;
};
template <class T>
T & List_iterator<T>::operator*()
{
return current_link -> value;
}
template <class T>
List_iterator<T> & List_iterator<T>::operator++()
{
current_link = current_link -> next_link;
return *this;
}
template <class T>
void List<T>::push_back(const T & x)
{
Link<T> * new_link = new Link<T> (x);
if (first_link = 0)
first_link = last_link = new_link;
else
{
new_link->prev_link = last_link;
last_link->next_link = new_link;
last_link = new_link;
}
my_size++;
}
template <class T>
typename List<T>::iterator List<T>::end() const
{
return iterator(last_link);
}
template <class T>
List <T>::~List()
{
Link <T> * first = first_link;
while (first != 0)
{
Link <T> * next = first->next_link;
delete first;
first = next;
}
}
template<class T>
bool List_iterator<T>::operator==(const iterator & rhs) const
{
return ( this->current_link == rhs.current_link );
}
template <class T>
bool List_iterator<T>::operator!=(const iterator & rhs) const
{
return !( *this == rhs );
}
int main()
{
List<int> l;
l.push_back(44); // list = 44
l.push_back(33); // list = 44, 33
l.push_back(11); // list = 44, 33, 11
l.push_back(22); // list = 44, 33, 11, 22
List<int> m(l);
List<int>::iterator itr(m.begin());
while (itr != m.end()) {
cout << *itr << endl;
++itr;
}
}

Answer 1

Mario's and Joe's answers are both valid (I would vote them up if I could).
Issue 1
Assuming that what you posted is your ACTUAL code for push_back() then Link<T> has the wrong case.
link<T> * last_link = new link<T> (x);
...should be:
Link<T> * last_link = new Link<T> (x);
Doing this gets rid of error: expected primary-expression before ‘>’ token (when compiled with g++)
Issue 2
An alternative to what Mario and Joe suggested, is to NOT hide your List<T>'s last_link. Use a temporary variable that does NOT have the same name as your class's data member. Change:
Link<T> * last_link = new Link<T> (x);
...to...
Link<T> * new_link = new Link<T> (x);
This will make your code clearer and you will be referencing the right node.
Issue 3
Following the above steps should make two logic errors in push_back() evident. I'm willing to assist, but I'm sure you will see it.

Answer 2

The following line creates a local variable named last_link inside push_back().
link<T> * last_link = new link<T> (x);
Referencing last_link after this will point to this local variable instead of the class member. You'll have to add this-> in front of the class ones, e.g. this->last_link = last_link;.

Answer 3

You have a local variable that is the same as a member variable name. When you're using last_link you're using the one that you just created in the push_back function. You're also assigning last_link->prev_link twice. Then also assigning last_link = last_link. I'd either change the name of the local variable, or add this-> in front of it.