queue::front() and queue::back() in C++ STL

Queue are a type of container adaptors which operate in a first in first out (FIFO) type of arrangement. Elements are inserted at the back (end) and are deleted from the front.

queue::front()

This function is used to reference the first or the oldest element of the queue container. This function can be used to fetch the first element of a queue.
Syntax :

queuename.front()
Parameters :
No value is needed to pass as the parameter.
Returns :
Direct reference to the first element of the queue container.

Examples: 

Input  :  myqueue = 1, 2, 3
          myqueue.front();
Output :  1

Input  :  myqueue = 3, 4, 1, 7, 3
          myqueue.front();
Output :  3

Errors and Exceptions

1. If the queue container is empty, it causes undefined behavior
2. It has a no exception throw guarantee if the queue is not empty

CPP

// CPP program to illustrate
// Implementation of front() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> myqueue;
    myqueue.push(3);
    myqueue.push(4);
    myqueue.push(1);
    myqueue.push(7);

    // Queue becomes 3, 4, 1, 7

    cout << myqueue.front();
    return 0;
}

Output:

3

queue::back()

This function is used to reference the last or the newest element of the queue container. This function can be used to fetch the first element from the back of a queue.
Syntax :

queuename.back()
Parameters :
No value is needed to pass as the parameter.
Returns :
Direct reference to the last element of the queue container.

Examples: 

Input  :  myqueue = 1, 2, 3
          myqueue.back();
Output :  3

Input  :  myqueue = 3, 4, 1, 7, 3
          myqueue.back();
Output :  3

Errors and Exceptions

1. If the queue container is empty, it causes undefined behavior
2. It has a no exception throw guarantee if the queue is not empty

CPP

// CPP program to illustrate
// Implementation of back() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> myqueue;
    myqueue.push(3);
    myqueue.push(4);
    myqueue.push(1);
    myqueue.push(7);

    // Queue becomes 3, 4, 1, 7

    cout << myqueue.back();
    return 0;
}

Output:

7

Application :
Given an empty queue of integers, add numbers to the queue, then print the difference between the first and the last element. 

Input  : 1, 2, 3, 4, 5, 6, 7, 8
Output : 7
Explanation - Last element = 8, First element = 1, Difference = 7

Algorithm
1. Add numbers to the queue using push() function 
2. Compare the first and the last element. 
3. If first element is larger, subtract last element from it and print it. 
4. Else subtract first element from the last element and print it.

CPP

// CPP program to illustrate
// application Of front() and back() function
#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> myqueue;
    myqueue.push(8);
    myqueue.push(7);
    myqueue.push(6);
    myqueue.push(5);
    myqueue.push(4);
    myqueue.push(3);
    myqueue.push(2);
    myqueue.push(1);

    // Queue becomes 1, 2, 3, 4, 5, 6, 7, 8

    if (myqueue.front() > myqueue.back()) {
        cout << myqueue.front() - myqueue.back();
    }
    else if (myqueue.front() < myqueue.back()) {
        cout << myqueue.back() - myqueue.front();
    }
    else
        cout << "0";
}

Output:

7

Let us see the differences in a tabular form -:

	queue::front()	queue::back()
1.	It is used to return a reference to the next element in the queue.	It is used to return a reference to the last element in the queue.
2.	Its syntax is -: front();	Its syntax is -: back();
3.	It does not take any parameters.	It does not take any parameters.
4.	Its complexity is constant.	Its complexity is constant.
5.	It is defined in <queue> header file.	It is defined in <queue> header file.