priority_queue::swap() in C++ STL

Priority queues are a type of container adaptors, specifically designed such that the first element of the queue is either the greatest or the smallest of all elements in the queue. However, in C++ STL (by default) the largest element is at the top. We can also create a priority queue having the smallest element at the top by simply passing an extra parameter while creating the priority queue.

priority_queue::swap()

This function is used to swap the contents of one priority queue with another priority queue of same type and same or different size.

Time complexity: O(1)

Syntax :

priorityqueuename1.swap(priorityqueuename2)
Parameters :
The name of the priority queue with which the contents have to be swapped.
Result :
All the elements of the 2 priority queues are swapped.

Examples:

Input  : mypqueue1 = {1, 2, 3, 4}
         mypqueue2 = {3, 5, 7, 9}
         mypqueue1.swap(mypqueue2);
Output : mypqueue1 = {9, 7, 5, 3}
         mypqueue2 = {4, 3, 2, 1}

Input  : mypqueue1 = {1, 3, 5, 7}
         mypqueue2 = {2, 4, 6, 8}
         mypqueue1.swap(mypqueue2);
Output : mypqueue1 = {8, 6, 4, 2}
         mypqueue2 = {7, 5, 3, 1}

Note: In priority_queue container, the elements are printed in reverse order because the top is printed first
      then moving on to other elements.

Errors and Exceptions 1. It throws an error if the priority queues are not of the same type. 2. It has a basic no exception throw guarantee otherwise. 

Example 1: It swaps two priority queue of same type and same size

CPP

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

int main()
{
    // priority_queue container declaration
    priority_queue<int> mypqueue1;
    priority_queue<int> mypqueue2;

    // pushing elements into the 1st priority queue
    mypqueue1.push(1);
    mypqueue1.push(2);
    mypqueue1.push(3);
    mypqueue1.push(4);

    // pushing elements into the 2nd priority queue
    mypqueue2.push(3);
    mypqueue2.push(5);
    mypqueue2.push(7);
    mypqueue2.push(9);

    // using swap() function to swap elements of priority queues
    mypqueue1.swap(mypqueue2);

    // printing the first priority queue
    cout << "mypqueue1 = ";
    while (!mypqueue1.empty()) {
        cout << mypqueue1.top() << " ";
        mypqueue1.pop();
    }

    // printing the second priority queue
    cout << endl
        << "mypqueue2 = ";
    while (!mypqueue2.empty()) {
        cout << mypqueue2.top() << " ";
        mypqueue2.pop();
    }
    return 0;
}

Output

mypqueue1 = 9 7 5 3 
mypqueue2 = 4 3 2 1 

Example 2: It swaps two priority queue of same type but different size

C++

// CPP program to illustrate
// Implementation of swap() function
// for different sizes

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // priority_queue container declaration
    priority_queue<int> mypqueue1;
    priority_queue<int> mypqueue2;

    // pushing elements into the 1st priority queue
    mypqueue1.push(1);
    mypqueue1.push(2);
    mypqueue1.push(3);
    mypqueue1.push(4);
    mypqueue1.push(5);

    // pushing elements into the 2nd priority queue
    mypqueue2.push(3);
    mypqueue2.push(5);
    mypqueue2.push(7);
    mypqueue2.push(9);

    // using swap() function to swap elements of priority queues
    mypqueue1.swap(mypqueue2);

    // printing the first priority queue
      cout<<"mypqueue1 = ";
    while (!mypqueue1.empty()) {
        cout<<mypqueue1.top() << " ";
        mypqueue1.pop();
    }

    // printing the second priority queue
      cout<<endl<<"mypqueue2 = ";
    while (!mypqueue2.empty()) {
        cout<<mypqueue2.top() << " ";
        mypqueue2.pop();
    }
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

mypqueue1 = 9 7 5 3 
mypqueue2 = 5 4 3 2 1