queue implementation with vector c++

Question

We were instructed to implement a class that uses a vector to store a queue. I came up with the following but it's not really working. Can anyone tell me what went wrong?

The values of the numbers are correctly pushed into the vec, and the first pop() works. But if I check head->getElement(), it gives a strange number. Subsequent calls to pop() also fail.

#include <iostream>
#include <vector>
using namespace std;

template<class T>
class node{
    T element;
    node* next;
public:
    node(): next(nullptr){};
    T getElement() {return element;}
    void setElement(T newElement) {element=newElement;}
    node* getNext() {return next;}
    void setNext(node* newNext) {next = newNext;}
};

template<class T>
class queue{
    vector<node<T>> vec;
    node<T>* head;
    int size;
public:
    queue(): head(nullptr){}
    void push(node<T> newNode);
    node<T> pop();
    int getSize() {return unsigned(vec.size());}
    vector<T> getVec()const {return vec;}
    node<T>* getHead() {return head;}
    void setHead(node<T>* newHead) {head = newHead;}
    vector<node<T>> getVec() {return vec;}
};

int main() {
    queue<int> v;

    for (int i=0;i<5;i++){
        node<int>* newNode = new node<int>;
        newNode->setElement(i);
        v.push(*newNode);
    }
    cout<<"The elements in the vector are initially:\n";
    for (int i=0; i<v.getSize();i++)
        cout<<v.getVec()[i].getElement()<<" ";
    cout<<"\nAfter popping, the popped element is "<<v.pop().getElement()<<endl;
}

template<class T>
node<T> queue<T>:: pop(){
    node<T>* tmp = new node<T>;
    tmp->setNext(head->getNext());
    head=head->getNext();
    return *tmp;
}

template<class T>
void queue<T>:: push(node<T> newNode){
    if (head==nullptr){
        node<T>* newPtr = new node<T>;
        newPtr = &newNode;
        newPtr->setNext(head);
        head=newPtr;
    }
    else{
        node<T>* newPtr = new node<T>;
        newPtr->setElement(newNode.getElement());
        node<T>* end = head;
        while (end->getNext() != nullptr)
            end->setNext(end->getNext());
        end->setNext(newPtr);
    }
    vec.push_back(newNode);
}

Answer 1

There are many issues with your code. First, lets fix pop. You are creating a new node tmp, setting the next and returning the same without setting the element. To fix this, you just need it to set it to head and move the head to its next.

template<class T>
node<T> queue<T>:: pop(){
node<T>* tmp = head;// = new node<T>;
//tmp->setNext(head->getNext());
head=head->getNext();
return *tmp;
}

After this you will get your element with pop. But you will get wrong element. As queue is FIFO based, you should get '0' at first pop but in your case you will not get 0. Because, your push function is also incorrect. In push, when you are pushing the first element, then you are taking the address of the node object passed which is passed by value, this leads to undefined behavior, since the node object passed would be destroyed once the function finishes. Also, in the else part of your push you are setting the next of end to its own next and hence it will go into infinite loop. Below is the corrected implementation.

template<class T>
void queue<T>:: push(node<T> newNode){
if (head==nullptr){
    node<T>* newPtr = new node<T>;
    newPtr->setElement(newNode.getElement());
    //newPtr = &newNode;
    //newPtr->setNext(head);
    head=newPtr;
}
else{
    node<T>* newPtr = new node<T>;
    newPtr->setElement(newNode.getElement());
    node<T>* end = head;
    while (end->getNext() != nullptr)
        end = end->getNext();
    end->setNext(newPtr);
}
vec.push_back(newNode);
}

Lastly, there are many memory leaks in your code. You are creating a new node far more number of times than needed and also not deleting them. As, you can see now that in case of push, you just need to pass the element T and it would suffice. You don't need to pass a new node every time. Also, try using smart pointers as it will manage a lot of things on its own.