How to insert elements from a structure into a min heap priority queue

Question

I am making a file compressor using Huffman Algorithm in C++. I have calculated the character frequency, but now I am having difficulty in pushing it into a min heap priority queue. I want to sort the inserted elements by frequency. Every time I execute the code, it gives the error:

Error C2676 binary '>': 'const _Ty' does not define this operator or a conversion to a type acceptable to the predefined operator

I have tried almost every way mentioned on this website, yet I still cannot get rid of this error.

Here is my code:

#include<iostream>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<algorithm>

using namespace std;

struct node
{
    char c; //character in the string
    int f; //Frequency of character in the string
    node* next;
    node* left, * right; //left and right child of binary tree respectively

    node()
    {
        f = 0;
        left = NULL;
        right = NULL;
        c = NULL;
        next = NULL;    
    }

    bool operator>(const node& a)
    {
        return a.f > f;
    }

    };

class Huffman
{
    string text; //The text that will be encoded
    priority_queue <node> pq;
public:
    Huffman()
    {
        
    }

    void StringInput()
    {
        cout << "Enter the string you want to encode:";
        getline(cin, text);
    }

    //Function which will calculate the frequency of characters in the string entered by the user
    void CharacterFrequency()
{
        
        for (int i = 0; i < text.length(); i++)
        {
            int sum = 0;
            for (int j = 0; j < text.length(); j++)
            {

                if (j < i and text[i] == text[j])
                {
                    break;
                }


                    if (text[i] == text[j])
                    {
                        sum++;
                    } 
                    
                    
                
            }

            if (sum != 0)
            {
                PriorityQueue(text[i], sum);
            }
        }
}

void PriorityQueue(char ch, int freq)
    {
        
        node n;
        n.c = ch;
        n.f = freq;
        pq.push(n);
        

        
    }


};

int main()
{
    Huffman obj;
    obj.StringInput();
    obj.CharacterFrequency();
    
    return 0;
}

I will be grateful for any help in this regard.

Answer 1

There are two issues:

Issue 1 : You are not declaring the priority queue correctly.

If you want to add a custom comparison, you need to specify that in the template:

std::priority_queue <node, std::vector<node>, std::greater<node>> pq;

Now the node::operator > will be used.

---

Issue 2 : The operator > should be a const function:

bool operator>(const node& a) const
{
    return a.f > f;
}