I can't associate private class members with a friendly function

Question

I have a problem with linking a class-friendly function and its private variables. The training task consists in creating a program that encrypts and decrypts the entered text based on the key with a Vigener cipher. Please help me! (here is link to dropmefiles and archive, where program places. *It's my first question in stackoverflow https://dropmefiles.com/JZFJd here there is code of declaration class with friendly-funcion ("Vigenere_square.h"):

#pragma once
#include <iostream>
#include <string>
#include <fstream>


class Vigenere_square
{
private:
    const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    size_t size;
    char** array;

public:
    Vigenere_square();
    ~Vigenere_square();
    //friend void encode();
    //friend void decode();
    //friend void output();
    friend string encode_string(const string& text, const string& key, const Vigenere_square& vs);
    
};

and here code of "encode.cpp" file:

#include "encode.h"
using namespace std;
string encode_string(const string& text, const string& key, const Vigenere_square& vs)
{
    string text2, key2;
    while (key2.length() <= text.length())
    {
        key2 += key;
    }
    size_t row, column, k;
    for (size_t k = 0; k < text.length(); k++) 
    {
        size_t row = vs.ALPHABET.find(key2[k]);
        size_t column = vs.ALPHABET.find(text[k]);
        if (row < vs.size && column < vs.size) {
            text2 += vs.array[row][column];
        }
        else {
            text2 += text[k];
        }
    }
    return text2;
}

I'm just a student, but I really don't know what I have to learn or understand. There is a very stupid mistake, but I can't find it.

Answer 1

You are missing header inclusion. Instead of #include "encode.h", use #include "Vigenere_square.h".

Friend functions may not be necessity here, just use public member functions.

If you need to expose private members, you can use setter/getter.