queue::push() and queue::pop() in C++ STL

Difficulty Level : Basic
Last Updated : 20 Jun, 2020

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::push()

push() function is used to insert an element at the back of the queue. The element is added to the queue container and the size of the queue is increased by 1.

Syntax :

queuename.push(value)
Parameters :
The value of the element to be inserted is passed as the parameter.
Result :
Adds an element of value same as that of 
the parameter passed at the back of the queue.

Examples:

Input :  myqueue
         myqueue.push(6);
Output : 6

Input :  myqueue
         myqueue.push(0);
         myqueue.push(1);
Output : 0, 1

Errors and Exceptions

Shows error if the value passed doesn’t match the queue type.
Shows no exception throw guarantee if the parameter doesn’t throw any exception.

// CPP program to illustrate 
// Implementation of push() function 
#include <iostream> 
#include <queue> 
using namespace std; 
  
int main() 
{ 
    // Empty Queue 
    queue<int> myqueue; 
    myqueue.push(0); 
    myqueue.push(1); 
    myqueue.push(2); 
  
    // Printing content of queue 
    while (!myqueue.empty()) { 
        cout << ' ' << myqueue.front(); 
        myqueue.pop(); 
    } 
} 

Output:

0 1 2

queue::pop()

pop() function is used to remove an element from the front of the queue(oldest element in the queue). The element is removed to the queue container and the size of the queue is decreased by 1.

Syntax :

queuename.pop()
Parameters :
No parameters are passed
Result :
Removes the oldest element in the queue
or basically the front element.

Examples:

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

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

Errors and Exceptions

Shows error if a parameter is passed.
Shows no exception throw guarantee if the parameter doesn’t throw any exception.

// CPP program to illustrate 
// Implementation of pop() function 
#include <iostream> 
#include <queue> 
using namespace std; 
  
int main() 
{ 
    // Empty Queue 
    queue<int> myqueue; 
    myqueue.push(0); 
    myqueue.push(1); 
    myqueue.push(2); 
    // queue becomes 0, 1, 2 
  
    myqueue.pop(); 
    myqueue.pop(); 
    // queue becomes 2 
  
    // Printing content of queue 
    while (!myqueue.empty()) { 
        cout << ' ' << myqueue.front(); 
        myqueue.pop(); 
    } 
} 

Output:

Application : push() and pop()
Given a number of integers, add them to the queue and find the size of the queue without using size function.

Input : 5, 13, 0, 9, 4
Output: 5

Algorithm
1. Push the given elements to the queue container one by one.
2. Keep popping the elements of queue until the queue becomes empty, and increment the counter variable.
3. Print the counter variable.

// CPP program to illustrate 
// Application of push() and pop() function 
#include <iostream> 
#include <queue> 
using namespace std; 
  
int main() 
{ 
    int c = 0; 
    // Empty Queue 
    queue<int> myqueue; 
    myqueue.push(5); 
    myqueue.push(13); 
    myqueue.push(0); 
    myqueue.push(9); 
    myqueue.push(4); 
    // queue becomes 5, 13, 0, 9, 4 
  
    // Counting number of elements in queue 
    while (!myqueue.empty()) { 
        myqueue.pop(); 
        c++; 
    } 
    cout << c; 
} 

Output:

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