priority_queue::swap() in C++ STL

Difficulty Level : Easy
Last Updated : 08 Aug, 2019

Priority queues are a type of container adaptors, specifically designed such that the first element of the queue is the greatest of all elements in the queue.

priority_queue::swap()

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

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.

// 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

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