priority_queue emplace() 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::emplace()

This function is used to insert a new element into the priority queue container, the new element is added to the priority queue according to its priority. It is similar to push operation. The difference is that emplace() operation saves unnecessary copy of the object.

Time complexity: O(log n)

Syntax :

priorityqueuename.emplace(value)
Parameters :
The element to be inserted into the priority
queue is passed as the parameter.
Result :
The parameter is added to the
priority queue at the top position.

Examples: 

Input  : mypqueue{5, 4};
         mypqueue.emplace(6);
Output : mypqueue = 6, 5, 4

Input  : mypqueue{};
         mypqueue.emplace(4);
Output : mypqueue = 4

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 has a strong exception guarantee, therefore, no changes are made if an exception is thrown. 
2. Parameter should be of same type as that of the container, otherwise an error is thrown.

C++

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

int main()
{
    priority_queue<int> mypqueue;
    mypqueue.emplace(1);
    mypqueue.emplace(2);
    mypqueue.emplace(3);
    mypqueue.emplace(4);
    mypqueue.emplace(5);
    mypqueue.emplace(6);
    // queue becomes 1, 2, 3, 4, 5, 6

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

    return 0;
}

Output

mypqueue = 6 5 4 3 2 1 

C++

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

int main()
{
    priority_queue<char> mypqueue;
    mypqueue.emplace('A');
    mypqueue.emplace('b');
    mypqueue.emplace('C');
    mypqueue.emplace('d');
    mypqueue.emplace('E');
    mypqueue.emplace('f');
    // queue becomes A, b, C, d, E, f

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

    return 0;
}

Output

mypqueue = f d b E C A 

C++

// STRING PRIORITY QUEUE
// CPP program to illustrate
// Implementation of emplace() function
#include <iostream>
#include <queue>
#include <string>
using namespace std;

int main()
{
    priority_queue<string> mypqueue;
    mypqueue.emplace("portal");
    mypqueue.emplace("computer science");
    mypqueue.emplace("is a");
    mypqueue.emplace("GEEKSFORGEEKS");
    // queue becomes portal, computer science,
    // is a, GEEKSFORGEEKS

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

    return 0;
}

Output

mypqueue = portal is a computer science GEEKSFORGEEKS 

Application :
Given a number of integers, add them to the priority queue using emplace() and find the size of the priority queue. 

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

Algorithm
1. Insert the given elements to the priority queue container one by one using emplace(). 
2. Keep popping the elements of priority queue until it becomes empty, and increment the counter variable. 
3. Print the counter variable.

C++

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

int main()
{
    int c = 0;

    // Empty Priority Queue
    priority_queue<int> pqueue;

    // inserting elements into priority_queue
    pqueue.emplace(5);
    pqueue.emplace(13);
    pqueue.emplace(0);
    pqueue.emplace(9);
    pqueue.emplace(4);

    // Priority queue becomes 13, 9, 5, 4, 0

    // Counting number of elements in queue
    while (!pqueue.empty()) {
        pqueue.pop();
        c++;
    }
    cout << c;
}

Output

5

emplace() vs push()
When we use push(), we create an object and then insert it into the priority_queue. With emplace(), the object is constructed in-place and saves an unnecessary copy. Please see emplace vs insert in C++ STL for details.

C++

// C++ code to demonstrate difference between
// emplace and insert
#include<bits/stdc++.h>
using namespace std;

int main()
{
    // declaring priority queue
    priority_queue<pair<char, int>> pqueue;

    // using emplace() to insert pair in-place
    pqueue.emplace('a', 24);

    // Below line would not compile
    // pqueue.push('b', 25);

    // using push() to insert pair
    pqueue.push(make_pair('b', 25));

    // printing the priority_queue
    while (!pqueue.empty()) {
        pair<char, int> p =  pqueue.top();
        cout << p.first << " "
             << p.second << endl;
        pqueue.pop();
    }

    return 0;
}

Output

b 25
a 24