call to non-static member function without an object argument, C++ nodes

Question

This appears to be covered in depth on this website but after searching for a while I can't seem to find what my problem specifically is, as I'm getting the error with a void function.

I'm attempting to code a Huffman Tree and I'm trying to build a min heap. In order to do that I need to be able to swap nodes in the tree. I keep receiving the above error on the line where I call swapNode for the first time.

Node.cpp offending code

void Node::swapNode(Node &a, Node &b) {
    Node t = a;
    a = b;
    b = t;
}
Node**minHeapify(Node **array, int index) { 
    int root = index;
    int left = 2 * index + 1;
    int right = 2 * index + 2;
    if (array[left]->frequency < array[root]->frequency) {
        root = left;
    }
    if (array[right]->frequency < array[root]->frequency) {
        root = right;
    } 
    if(root != index) {
        Node::swapNode(&array[root], &array[index]);
        minHeapify(array, root);
    }
}

Node.h

#ifndef _listnode_h
#define _listnode_h

#include <string>

class Node {
    public: 
        Node(char character, int frequency, int ascii);
        Node(int frequency);
        Node();
        ~Node();
        int getFrequency();
        char getCharacter();
        std::string getCode();
        Node *getLeft();
        Node *getRight();
        Node **minHeapify(Node *, int);
        void swapNode(Node &a, Node &b);    
        void setCode(std::string s);

//  private:
        int frequency;
        char character; 
        int ascii;
        std::string code;
        int leaf = 0;
        Node* left, *right; 
};

#endif 

The array I'm passing in is an array of tree nodes that I want to sort into a min heap. Any and all help is appreciated, thanks.

Answer 1

If you want to call swapNode without invoking it directly on an actual object (which is reasonable) and you want it to be a class member, then you need to declare the function static. Or you can just use std::swap