C++ iterating nodes with respect to encapsulation - c++

Question

I'm writing a function which iterates a Queue from within a queue class which operates off of a LinkedList/Node data structure.
I've been able to make the function work but only by getting a pointer to the head node directly from the LinkedList class which, as I understand it, is considered poor encapsulation.
This is my code:
main():
int main()
{
Queue list;
int nums[] = {60, 50, 40};
for (int i=0; i<(int)sizeof(nums)/(int)sizeof(nums[0]); i++) {list.enqueue(nums[i]);}
list.iterate();
}
Queue:
.h
#include "LinkedList.h"
class Queue
{
public:
typedef int value_type;
Queue();
void enqueue(value_type& obj);
int size() const;
void iterate();
int min();
private:
LinkedList data;
int used;
};
#include "Queue.hpp"
.hpp
Queue::Queue()
{ data = LinkedList(); used = 0; }
void Queue::enqueue(value_type& obj)
{ ++used; data.addToTail(obj); }
int Queue::size() const
{ return used; }
void Queue::iterate()
{
node * temp = data.get_head();
for (int i = 0; i < size(); i++)
{ cout << temp->get_data() << endl; temp = temp->get_next(); }
delete temp;
}
LinkedList
.h
#include "Node.h"
class LinkedList
{
public:
typedef int value_type;
LinkedList();
void addToHead(typename node::value_type& entry);
void addToTail(typename node::value_type& entry);
node * get_head();
int front();
private:
node* head;
node* tail;
node* current;
};
#include "LinkedList.hpp"
.hpp
LinkedList::LinkedList()
{ head = NULL; tail = NULL; current = NULL; }
void LinkedList::addToTail(value_type& entry)
{
if (get_head() == NULL)
{ addToHead(entry); }
else {
node* add_ptr = new node;
add_ptr->set_data(entry);
add_ptr->set_next(current->get_next());
add_ptr->set_previous(current);
current->set_next(add_ptr);
if (current == tail) {tail = current->get_next();}
current = current->get_next();
}
}
void LinkedList::addToHead(value_type& entry)
{ head = new node(entry, head); if (tail == NULL) {tail = head;} current = head; }
node * LinkedList::get_head()
{ return head; }
int LinkedList::front()
{ int rval = head->get_data();return rval; }
Node
.h
class node
{
public:
typedef int value_type;
node();
node(const value_type& data, node* link);
void set_data(const value_type& new_data);
void set_next(node* next_ptr);
void set_previous(node* last_ptr);
int get_data() const;
node* get_next() const;
node* get_previous() const;
private:
value_type data;
node* next;
node* previous;
};
#include "Node.hpp"
.hpp
node::node()
{ data = 0; next = 0; previous = 0; }
node::node(const value_type& data, node* link)
{ this->data = data; this->next = link; this->previous = NULL; }
void node::set_data(const value_type& new_data) {data = new_data;}
void node::set_next(node* next_ptr) {next = next_ptr;}
void node::set_previous(node* last_ptr) {previous = last_ptr;}
int node::get_data() const {return data;}
node* node::get_next() const {return next;}
node* node::get_previous() const {return previous;}
Is it possible to iterate the LinkedList without directly retrieving a pointer node? And is this bad practice?

Answer 1

You do not expose the (internal) data structures of the linked list within the interface of the Queue-class (i.e. in the header file). You're just using these data structures in the implementation. Hence, I'd say that you do not "violate encapsulation".
But of course, you may adapt the interface of your LinkedList, such that it does not make use of the internal data structures directly. The standard library with its iterators shows how such a concept is realized. An iterator is an object that represents the position of an element in the container, (and it offers access to the respective element).

Answer 2

The encapsulation in Queue isn't violated but in LinkedList it is, you shouldn't have get_head() function that returns a private pointer member (what if someone does something like this: list.get_head()->set_next(NULL)). You need to create an iterate function in LinkedList and than Queue::iterate would just call this function.