How to Use priority_queue in C++: Syntax and Examples

In C++, use std::priority_queue to store elements in a way that the largest element is always accessible at the top. It works like a max-heap by default, and you can push elements with push(), access the top with top(), and remove it with pop().

Syntax

The std::priority_queue is a container adapter that provides a priority queue interface. It is declared as:

std::priority_queue<Type, Container, Compare> pq;

Where:

• Type is the data type stored.
• Container is the underlying container type (default is std::vector<Type>).
• Compare is the comparison function object that defines priority (default is std::less<Type> for max-heap).

cpp

std::priority_queue<int> pq; // max-heap of integers by default

Example

This example shows how to create a priority queue, add elements, access the top element, and remove elements in order of priority.

cpp

#include <iostream>
#include <queue>

int main() {
    std::priority_queue<int> pq;

    pq.push(10);
    pq.push(30);
    pq.push(20);
    pq.push(5);

    std::cout << "Elements in priority order:" << std::endl;
    while (!pq.empty()) {
        std::cout << pq.top() << " ";
        pq.pop();
    }
    std::cout << std::endl;
    return 0;
}

Output

Elements in priority order:
30 20 10 5 

Common Pitfalls

Common mistakes when using priority_queue include:

• Trying to iterate over the queue directly (it does not support iterators).
• Assuming it sorts all elements (it only guarantees the top is the highest priority).
• Not specifying a custom comparator correctly for min-heap behavior.

To create a min-heap, you must use std::greater<Type> as the comparator and specify the container:

cpp

#include <queue>
#include <vector>
#include <functional>

// Correct way to create min-heap:
std::priority_queue<int, std::vector<int>, std::greater<int>> min_pq;

// Wrong: Trying to iterate directly over pq (no iterators)
// for (auto x : pq) { /* error */ }

Quick Reference

Operation	Description	Example
push	Add element to the queue	pq.push(5);
top	Access highest priority element	int x = pq.top();
pop	Remove highest priority element	pq.pop();
empty	Check if queue is empty	if (pq.empty()) {...}
size	Get number of elements	int n = pq.size();

Key Takeaways

Use std::priority_queue to always access the largest element quickly by default.

Push elements with push(), access the top with top(), and remove it with pop().

To create a min-heap, specify std::greater as the comparator and vector as the container.

You cannot iterate directly over a priority_queue; only access top element.

Remember priority_queue only guarantees the top element priority, not full sorting.