如何将类的公共成员设为私有

我对编程(一般来说)和c++(特别是)是一个全新的人。我试图采取以下公共成员变量，并使它们私有:

int *coeff;
int order;

不幸的是，我看到了以下错误:

'Poly::coeff':不能访问在'Poly'类中声明的私有成员

和

'Poly::order':不能访问在'Poly'类中声明的私有成员

下面是我的代码:

#include "stdafx.h"
#include <iostream>
#include <cstdlib>
using namespace std;
class Poly
{
public:
    int *coeff;
    int order;
    int getData();
    int display(int *coeff, int order);
    void addition(Poly P1, Poly P2);
    void subtraction (Poly P1, Poly P2);
    void multiplication (Poly P1, Poly P2);
//  ~Poly();
};
int Poly::display(int *coeff, int order)
{
    int i;
    int j;
    for (i = order; i >= 0; i--)
    {
        cout << coeff[i] << "x^" << i;
        if ((i - 1) != -1)
        {
            cout << "+";
        }
    }
    cout << "n";
    return 0;
}
int Poly::getData()
{
    int i;
    cout << "Please enter the order of the polynomial: ";
    cin >> order;
    coeff = new int[order + 1];
    for (i = order; i >= 0; i--)
    {
        cout << "Please enter the coefficient of x^" << i << " :";
        cin >> coeff[i];
    }
    return 0;
}
void Poly::addition(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    int *add = new int[max + 1];
    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }
    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            add[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }
    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            add[i] = P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            add[i] = P1.coeff[i] + P2.coeff[i];
        }
    }
    cout << "nAddition:";
    display(add, max);
    cout << "n";
}
void Poly::subtraction(Poly P1, Poly P2)
{
    int max;
    int i;
    max = (P1.order > P2.order) ? P1.order : P2.order;
    int *sub = new int[max + 1];
    if (P1.order == P2.order)
    {
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }
    if (P1.order > P2.order)
    {
        for (i = P1.order; i > P2.order; i--)
        {
            sub[i] = P1.coeff[i];
        }
        for (i = P2.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }
    if (P1.order < P2.order)
    {
        for (i = P2.order; i > P1.order; i--)
        {
            sub[i] = -P2.coeff[i];
        }
        for (i = P1.order; i >= 0; i--)
        {
            sub[i] = P1.coeff[i] - P2.coeff[i];
        }
    }
    cout << "nSubtraction:";
    display(sub, max);
    cout << "n";
}
void Poly::multiplication(Poly P1, Poly P2)
{
    int i;
    int j;
    int max;
    max = P1.order + P2.order;
    int *mult = new int[max + 1];
    for (i = P1.order; i >= 0; i--)
    for (j = P2.order; j >= 0; j--)
    {
        mult[i + j] += P1.coeff[i] * P2.coeff[i];
    }
        cout << "nMultiplication:";
        display(mult, max);
}

int main()
{
    int choice;
    Poly P1, P2, P3;
    cout << "-------- Instructions --------" << endl;
    cout << "For polynomial 1... " << endl;
    P1.getData();
    cout << endl;
    cout << "For polynomial 2... " << endl;
    P2.getData();
    while (1)
    {
        cout << "n******** Menu Selection ********" << endl;
        cout << "1: Additionn2: Subtractionn3: Mutiplicationn0: Exit" << endl;
        cout << "Please enter your choice (1, 2, 3 or 0):";
        cin >> choice;
        switch (choice)
        {
        case 1:
            cout << "n-------- Addition --------n";
            cout << "Polynomial 1: ";
            P1.display(P1.coeff, P1.order);
            cout << "Polynomial 2: ";
            P2.display(P2.coeff, P2.order);
            P3.addition(P1, P2);
            cout << "--------------------------n";
            break;
        case 2:
            cout << "n------ Subtraction ------n";
            cout << "Polynomial 1: ";
            P1.display(P1.coeff, P1.order);
            cout << "Polynomial 2: ";
            P2.display(P2.coeff, P2.order);
            P3.subtraction(P1, P2);
            cout << "--------------------------n";
            break;
        case 3:
            cout << "n------ Subtraction ------n";
            cout << "Polynomial 1: ";
            P1.display(P1.coeff, P1.order);
            cout << "Polynomial 2: ";
            P2.display(P2.coeff, P2.order);
            P3.multiplication(P1, P2);
            cout << "--------------------------n";
            break;
        case 0:
            cout << "The program will now terminate.  Thank you." << endl;
            exit(0);
        default:
            cout << endl;
            cout << "You have entered an invalid selection." << endl;
            cout << "Please enter a positive integer between 0 and 3.";
            cout << endl;
        }
    }
    return 0;
}

事先非常感谢。对