c++ queue implementation not working as expected

Question

Today I am looking to make my own dequeue in c ++ using pointers. Well, in the program, I want to create two dequeues in which to add different elements. Unfortunately, this is not what happened. On screen i got 6 5 insted only 6. Why?I'm not sure exactly how to make two separate dequeues? I think one of my problems are *L = NULL, * R = NULL but i am not sure? So my code so far -

#include <iostream>

struct elem
{
    int key;
    elem* next;
}*L = NULL, * R = NULL;

void push_l(int n, elem*& p)
{
    p = L;
    L = new elem;
    L->key = n;
    L->next = p;

    if (R == NULL)
        R = L;
}

int pop_l(int& n, elem*& p)
{
    if (L)
    {
        p = L;
        n = L->key;
        L = L->next;

        if (L == NULL)
            R = NULL;
        delete p;

        return 1;
    }
    else
    {
        return 0;
    }
}



int main()
{
    int k;
    elem* p = new elem;
    elem* q = new elem;
    push_l(5, p);
    push_l(6, q);

    while (pop_l(k, q))
    {
        std::cout << k<< " ";
    }
}

I am not sure how can i print only 6 on the screen because this element is only pushed to the dequeue ? Also these are just some of the features for dequeue because I want to understand it from the beginning !

I would suggest starting by making a class called `dequeue` (or similar), which actually has the implementation. You are relying on global variables currently, thus both of your elements are using the same variables. — ChrisMM, May 13 '22 at 14:32
*'I'm not sure exactly how to make two separate dequeues?'* – drop global variables, make a class having head and tail pointers and the push- and pop-functions member functions of said class. Then you can instantiate the class multiple times. — Aconcagua, May 13 '22 at 14:33
It can help you to understand pointer arithmetics better if you draw on paper how your pointers are organised after every operation. You could e.g. draw squares for objects, circles for pointers and arrows to indicate where the pointers point to. — Aconcagua, May 13 '22 at 14:37
@BenjaminJones `struct` or `class` doesn't matter at all in C++, they are both *exactly* the same apart from default visibility of members and base classes... Quite a common convention is having a `class` for objects with functionality (which would be your case) and `structs` for POD types – but as said, nothing more than a convention. — Aconcagua, May 13 '22 at 14:38
You might want to peek into a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list)... — Aconcagua, May 13 '22 at 15:00

Aconcagua · Accepted Answer · 2022-05-13T14:56:09.837

Such a class might look like this:

class /* or struct, if you really want to... */ Dequeue 
{
public:
   void push(int value);
   bool pop(int& value); // if you return 1 for success and 0 for error
                         // bool is the more appropriate type...

private:
   struct elem // it's an implementation detail, so recommendable
               // to have it as a nested class...
   {
        // just as you had before...

        // you might additionally provide a constructor:
        // this makes creating new instances easier/simpler
        elem(int k, elem* n) : key(k), next(n) { }
   };

   elem* m_head = nullptr;
   elem* m_tail = nullptr; 
}

void Dequeue::push(int value)
{
//   auto tmp = new elem;
//   tmp->key = value;
//   tmp->next = m_head;
//   m_head = tmp;

// with constructor:
   m_head = new elem(value, m_head);

   if(!m_tail)
       m_tail = m_head;
}

bool Dequeue::pop(int& value)
{
   // transform analogously using m_head and m_tail instead of L and R
   // return true and false instead of 1 and 0 
}

You'll notice that the function looks just as before – however as now a member function, it accesses the class member of the instance upon which the function is called:

Dequeue d1; 
Dequeue d2;
// each of these have their own, separate m_head and m_tail pointers!

d1.push(12);
d2.push(10);
// now pushed to separate instances

Side note: untested code, if you find a bug, please fix yourself...

c++ queue implementation not working as expected

1 Answers1