Implementing a Stack in C++ using a linked list

Question

So I am trying to implement a Stack using a linked list and classes. Right now I have 6 different files: node.h, node.cpp, LL.h, LL.cpp, Stack.h, and Stack.cpp. I want to complete the Stack.h and Stack.cpp files so that they work as they should. I already implemented the linked list functions and they work as they should. Here is the code:

node.h :

// node.h

class node { // node class used in the LL (linked list) class
    private:
        node * next; // Pointer to next node of an LL
        int data;    // integer data stored in this node

    public:
        node(int x, node * n);  // Constructor
        ~node();                // Destructor
        void set_data(int x);   // Change the data of this node
        void set_next(node * n);// Change the next pointer of this node
        int get_data();         // Access the data of this node
        node * get_next();      // Access the next pointer of this node
};  

LL.h :

// LL.h
#include "node.h"

// Linked list class, used in the Stack class
class LL {
    private:
        node * head; // pointer to first node
        node * tail; // pointer to last node

    public:
        LL(); // Constructor
        ~LL(); // Destructor
        void prepend(int value); // add a node to the beginning of the LL
        int removeHead();        // remove the first node of the LL
        void print();            // print the elements of the LL
    node * get_head();       // access the pointer to the first node of the LL
};

Stack.h:

// Stack.h
#include "LL.h"

class Stack {
private:
    LL_t intlist;

public:

    Stack();    // Constructor
    ~Stack();       // Destructor

    void push(int value);
    int pop();      
    int isEmpty();
    void print();
};

And lastly, Stack.cpp:

// Stack.cpp
#include "Stack.h"
#include <stdio.h>

Stack::Stack() {
    head= NULL;
    tail= NULL;
}

Stack::~Stack() {
    delete intlist;
}

int Stack::isEmpty() {
    return (head==NULL);
}

void Stack::push(int value) {

    head= value;
}

int Stack::pop() {

    if ( !isEmpty() ) {
        int temp= tail->get_data();
        delete tail;
        return temp;    
    }
    return -1;
}

I am having compiling issues. It says get_data() is undefined and "head" and "tail" is undefined, even though I have " #include "LL.h" " in Stack.h and in LL.h, I have "#include "node.h" ", so they all build on one another so it should work correct? I want it to compile so I can see if I am implementing Stack.h and Stack.cpp correctly. Do you see any issues with the way I am implementing them? If so, can you point them out? Also, any idea as to why I am getting these compiling issues? Any help appreciated!

Answer 1

Let's look at your actual questions

Stack::Stack() {
    head= NULL;
    tail= NULL;
}

results in error "head" and "tail" is undefined . Now look at the header files, where are the declarations of head and tail ? Answer, in the LL class not the Stack class. It's the responsibility of the LL class to initialise head and tail which it does the in the LL class default constructor. So your Stack constructor should look like this

Stack::Stack() {
}

Whenever you have a constructor for a class which contains another class a constructor for the other class will be called. In the case of Stack the default constuctor for LL is called implicitly, and this initialises head and tail for you. You don't have to do anything.

Now lets look at some more of your implementation.

Stack::~Stack() {
    delete intlist;
}

intList is not a pointer, so it cannot be deleted. It's clear that you are trying to 'call` the destructor for your list, but just like the constructor this happens automatically. Your destructor should look like this

Stack::~Stack() {
}

Or you could (probably should) just remove it completely.

Moving on

int Stack::isEmpty() {
    return (head==NULL);
}

Again you are trying to access head somewhere it isn't accessible. Your Stack class has an LL intlist object and that's what it should use, so (for instance)

int Stack::isEmpty() {
    return intlist.get_head() == NULL;
}

Smae thing here

void Stack::push(int value) {
    head= value;
}

should be

void Stack::push(int value) {
    intlist.prepend(value);
}

Use the object that the stack has (the intlist ) not the internals of other objects.

I'll leave you do sort out the rest. But you must understand the division of responsbilities that exist in your class design. The Stack class should not (and cannot) concern itself with the internals of the LL class. All that the operations that Stack needs to perform should be doable with the public interface of the LL class. If not then it's the LL class that needs to change.

Also note that your pop implementation is not just wrong in executuion it's wrong in concept. Pop should remove the head of the list, not the tail. A stack is a LIFO list (last in, first out) so pop removes the most recentaly added item. Now looking at the LL class there is a removeHead method (hint, hint).