Question

I want to implement Dijkstra algorithm and in serious need for storing vertex in a queue .

#include <iostream>
#include <queue>
using namespace std;

int main ()
{

priority_queue<int> mypq;//I want to put a pointer to vertex instead of int

mypq.push(10);//Here I want to push vertex 
mypq.push(20);
mypq.push(15);

cout << "mypq.top() is now " << mypq.top() << endl; 

return 0;
}

Read the commented section.

Answer 1

The main thing to keep in mind is that a priority_queue is a sorted container, so it requires that you define a comparison for the objects being stored (which must follow a strict, weak ordering).

Since you talk about Dijkstra’s algorithm, let’s assume each vertex has a weight, and we want the vertices ordered by those weights.

struct vertex { 
    int x, y;
    unsigned weight;

    vertex(int x, int y, unsigned weight) : x(x), y(y), weight(weight) {}
    bool operator <(vertex &other) { return weight < other.weight; }
};

Now a priority_queue of vertex objects is pretty easy:

std::priority_queue<vertex> vertices;

vertices.push(vertex(1, 2, 3));
vertices.push(vertex(0, 1, 2));
vertices.push(vertex(10, 11, 12));

std::cout << "Top = " << vertices.top() << "\n";

Edit: You’ll need to define an insertion operator for that last line to work — something like:

std::ostream &operator<<(std::ostream &os, vertex const &v) { 
    return os << "(" << v.x << ", " << v.y << '[' v.weight << "])\n";
}