Queue with Limited Size of Arrays, C++ (gcc)

Queue with Limited Size of Arrays

// Implement Queue with Limited Size of Arrays
// https://www.cs.bu.edu/teaching/c/queue/array/types.html

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

class ListNode{
public:
    int fixNum; 
    int used; 
    int read; 
    int write; 
    vector<int> v; 
    ListNode* next;
    ListNode* pre; 
public:
    ListNode(int num): fixNum(num), used(0), next(NULL), pre(NULL){
        v = vector<int>(num,0); // 
        read = write = 0; 
    }
    void push(int val){ 
        if(used >= fixNum) return; 
        v[write] = val;
        used++;
        write = (write+1)%fixNum; 
    }
    
    void pop() {
        if(used==0) return; 
        used--;
        read = (read+1)%fixNum; // NOTE
    };
    
    int front(){
        return v[read];
    }; 
    bool empty(){
        return used == 0; 
    }
    bool full(){
        return used == fixNum; 
    }
    
};

class QueueFixed{
public:
    int fixNum; 
    int used; 
    ListNode* front; 
    ListNode* back; 
public:
    QueueFixed(int num): fixNum(num), used(0){
        front = back = new ListNode(num);
    }
   
    void push(int v) {
        if(back->full()){
            back->pre = new ListNode(fixNum);
            back->pre->next = back; 
            back = back->pre;
        }
        back->push(v); 
        used++;
    }
    void pop() {
        if(used ==0) return; 
        if(front->empty()) {
            ListNode* p = front->pre;
            delete front;
            front = p; 
        }
        front->pop();
        used--; 
    }
    
    int get_front(){
        if(used == 0) return -1; 
        return front->front(); 
    }
    bool empty(){
        return used == 0; 
    }
    void print(){
        cout << "used = " << used << endl;
        cout << "front->used = " << front->used << endl;
        cout << "back->used = " << back->used << endl;
    }
};

int main()
{
    QueueFixed q(3); 
    q.push(1); q.push(2); q.push(3);
    q.print(); 
    
    //testing push()
    q.push(4);  q.push(5);
    q.print();
    
    //test front(), pop()
    cout << "=> q.front() = "<< q.get_front() << endl;
    q.pop(); 
    cout << "=> q.front() = "<< q.get_front() << endl;
    q.print(); 
    
}


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