In C++, a priority queue is a type of container in the STL (Standard Template Library) that is used to store elements with associated priorities. The elements are stored in the queue in such a way that the element with the highest priority is always at the front of the queue and the element with the lowest priority is always at the back.

To create a priority queue in C++, you need to include the <queue> header file. The priority queue is declared as follows:

#include <queue>

std::priority_queue<int> pq; // creates an empty priority queue of integers

This creates an empty priority queue of integers. To create a priority queue of a different type, simply replace the int with the desired data type.

By default, the priority queue uses the less-than operator (<) to compare elements and determine their priorities. If you want to use a different comparison operator or priority function, you can pass a comparison function as a second argument to the priority queue constructor. For example:

#include <queue>

bool compare(int a, int b) {
    return a > b;
}

std::priority_queue<int, std::vector<int>, decltype(compare)*> pq(compare); // creates a priority queue of integers using the "compare" function to determine priorities

In this example, we pass the compare function as the second argument to the priority queue constructor to specify that we want to use the function to determine the priorities of the elements. Note that we also need to specify the data type of the comparison function pointer (in this case, decltype(compare)*) as the third template argument of the priority queue.