Class member functions as friends in other classes C++

Question

I have to classes that represent random generators.

r_6.h

#include <vector>
#pragma once

class R_G_6 {
public:
    R_G_6() {};
    float getNextRand();
    void countFrequency();
    void printFrequency(vector<long>);
    vector<long>frequencies;
private:
    R_G_1 r_1;
    float sum;
    static const long m = 0;
    static const long sigma = 1;
};

r_6.cpp

#include r_6.h
float  R_G_6::getNextRand()
{
    sum = 0;
    int count = 0;
    while (count != 12)
    {
        sum += r_1.getNextRand();
        ++count;
    }
    return (m + (sum - 6)*sigma);
}

void R_G_6::countFrequency()
{
    vector<long>frequencies;
    for (int j = 0; j < 7; ++j)
        frequencies.push_back(0);
    const long maxRands = 1000;
    for (int i = 0; i < maxRands; ++i)
    {
        int r = getNextRand();
        frequencies[r] = frequencies[r] + 1;
    }
}

void R_G_6::printFrequency(vector<long>frequencies)
{
    if (frequencies.empty())
        cout << "You haven`t count frequency!" << endl;
    else
    {
        for (int i = -3; i < 3; ++i)
        {
            cout << "[" << i << ";" << i+1 << "]"
                << " | " << frequencies[i] << endl;
        }
    }
}

r_7.h

#include "r_1.h"
#include "r_6.h"
#include <vector>
#include <utility>
#pragma once

class R_G_7  {
public:
    R_G_7() {};
    pair<float, float> getNextRand();
    void countFrequency();
    friend void R_G_6::printFrequency(vector<long>);
    vector<long>frequencies;
private:
    R_G_1 r1;
};

r_7.cpp

pair<float, float> R_G_7::getNextRand()
{
    float v1, v2, s;
    while (true) {  
        v1 = 2 * (r1.getNextRand()) - 1;
        v2 = 2 * (r1.getNextRand()) - 1;
         s = v1*v1 + v2* v2;
        if (s < 1) break;
    }
    float x1, x2;
    x1 = v1*(sqrt((-2)*log(s) / s));
    x2 = v2*(sqrt((-2)*log(s) / s));
    return make_pair(x1, x2);
}

void R_G_7::countFrequency()
{
    for (int j = 0; j < 7; ++j)
        frequencies.push_back(0);
    const long maxRands = 1000;
    for (int i = 0; i < maxRands; i+=2)
    {
        pair<float, float> pair_num = getNextRand();
        int r1 = (pair_num.first);
        frequencies[r1] = frequencies[r1] + 1;
        int r2 = (pair_num.second);
        frequencies[r2] = frequencies[r2] + 1;
    }
}

I do not think that these two classes could or should be inherited from each other. And I do not wont to copy-paste printFrequency(vector<long>frequencies) function. So I thought that it could be a good idea to make it as friend. But in practice I can not use it for an object of type R_G_7:

void rand_gen_6()
{
    R_G_6 r6;
    r6.countFrequency();
    r6.printFrequency(r6.frequencies); // it is OK
}

void rand_gen_7()
{
    R_G_7 r7;
    r7.countFrequency();
    // this is not OK
    // r7.printFrequency(r7.frequencies);  
    // or r7.R_G_6::printFrequency(r7.frequencies);
}

Maybe someone know what`s wrong with my code or how to fix my problem.

Why don't you just make the printFrequency function something that isn't tied to the classes? Or why don't you create a base class with a virtual printFrequency method that has a base definition. Then, have the two random generators inherit that. There's a lot of ways you could go about doing this. — spektr, Oct 23 '15 at 15:43
Sorry, but I dont understand your mumbo jumbo. Why do you even consider inheritance? Why you cannot use it "in practice for an object of type R_G_7"? Why do you consider `friend`s? What I see is just a function that would best be a simple free function, as it is not using any data of the class but only works on the input passed to it — 463035818_is_not_an_ai, Oct 23 '15 at 15:51

score 2 · Accepted Answer · answered Oct 23 '15 at 15:56

2

The printFrequency method doesn't need any of the class member. So it could be put out of the class definition, and share it with both classes.

answered Oct 23 '15 at 15:56

Dorian

490
2
10

Class member functions as friends in other classes C++

r_6.h

r_6.cpp

r_7.h

r_7.cpp

1 Answers1