Queue using Stack.txt

#include <iostream>
#include <stack>
#include <stdexcept>
using namespace std;

class QueueUsingStacks{
  private:
    stack<int> stack1;
    stack<int> stack2;
    
    void transferStacks(){
      while(!stack1.empty()){
        stack2.push(stack1.top());
        stack1.pop();
      }
    }
    
  public:
    void enqueue(int value){
      stack1.push(value);
    }
    
    void dequeue(){
      if(stack2.empty()){
        if(stack1.empty()){
          underflow_error("queue is empty");
        }
        transferStacks();
      }
      stack2.pop();
    }
    
    bool isEmpty(){
      return stack1.empty() && stack2.empty();
    }
    
    int front(){
      if(stack2.empty()){
        if(stack1.empty()){
          underflow_error("queue is empty");
        }
        transferStacks();
      }
      return stack2.top();
    }
};

int main(){
  QueueUsingStacks s;
  s.enqueue(5);
  s.enqueue(12);
  s.enqueue(51);
  
  cout << "front element is : " << s.front();
  
}