## QUEUES:

Queues are a type of container adaptor, specifically designed to operate in a FIFO context (first-in first-out), where elements are inserted into one end of the container and extracted from the other.

queues are implemented as containers adaptors, which are classes that use an encapsulated object of a specific container class as its underlying container, providing a specific set of member functions to access its elements. Elements are pushed into the “back” of the specific container and popped from its “front”.

The underlying container may be one of the standard container class template or some other specifically designed container class. This underlying container shall support at least the following operations:

- empty
- size
- front
- back
- push_back
- pop_front

The standard container classes deque and list fulfill these requirements. By default, if no container class is specified for a particular queue class instantiation, the standard container deque is used.

### C++ Program for Queue implementation in STL

#include <iostream> #include <queue> #include <string> #include <cstdlib> using namespace std; int main() { queue<int> q; int choice, item; while (1) { cout<<"n---------------------"<<endl; cout<<"Queue Implementation in Stl"<<endl; cout<<"n---------------------"<<endl; cout<<"1.Insert Element into the Queue"<<endl; cout<<"2.Delete Element from the Queue"<<endl; cout<<"3.Size of the Queue"<<endl; cout<<"4.Front Element of the Queue"<<endl; cout<<"5.Last Element of the Queue"<<endl; cout<<"6.Exit"<<endl; cout<<"Enter your Choice: "; cin>>choice; switch(choice) { case 1: cout<<"Enter value to be inserted: "; cin>>item; q.push(item); break; case 2: item = q.front(); q.pop(); cout<<"Element "<<item<<" Deleted"<<endl; break; case 3: cout<<"Size of the Queue: "; cout<<q.size()<<endl; break; case 4: cout<<"Front Element of the Queue: "; cout<<q.front()<<endl; break; case 5: cout<<"Back Element of the Queue: "; cout<<q.back()<<endl; break; case 6: exit(1); break; default: cout<<"Wrong Choice"<<endl; } } return 0; }

### Sample Output:

--------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 9 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 8 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 7 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 6 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 5 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 1 Enter value to be inserted: 4 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 3 Size of the Queue: 6 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 4 Front Element of the Queue: 9 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 5 Back Element of the Queue: 4 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 2 Element 9 Deleted --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 3 Size of the Queue: 5 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 4 Front Element of the Queue: 8 --------------------- Queue Implementation in Stl --------------------- 1.Insert Element into the Queue 2.Delete Element from the Queue 3.Size of the Queue 4.Front Element of the Queue 5.Last Element of the Queue 6.Exit Enter your Choice: 6 ------------------