如何将类的公共成员设为私有？

Question

我对编程（一般而言）和 C++（特别是）是全新的。我正在尝试采用以下公共成员变量并将它们设为私有：

int *coeff;
int order;

很遗憾，我看到以下错误：

'Poly::coeff' : 无法访问在类 'Poly' 中声明的私有成员

和

'Poly::order' : 无法访问在类 'Poly' 中声明的私有成员

这是我的代码：

#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: Addition\n2: Subtraction\n3: Mutiplication\n0: 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;
}

谁能提供有关如何最好地修改我的代码以使 int *coeff 和 int order 成为私有成员的指导？

提前非常感谢。 -瑞安

Answer 1

制作private 的想法意味着您不允许在成员或friend 函数之外访问（这就是private 的目的）。代码在main 中失败，例如P1.display(P1.coeff, P1.order)，因为main 不是Poly 类的成员，因此不允许 访问Poly::coeff 或Poly::order（正如编译器告诉你的那样）。

我建议您将Poly::display(int *coeff, int order) 的定义更改为不需要您传入coeff 和order，因为实例已经知道这些值是什么。我也会拿出return 0，因为它不会给你买任何东西。

void Poly::display()
{
    int i;
    int j;
    for (i = order; i >= 0; i--)
    {
        cout << coeff[i] << "x^" << i;
        if ((i - 1) != -1)
        {
            cout << "+";
        }
    }
    cout << "\n";
}

在该成员函数中，变量 order 和 coeff 隐式地获取了您的类的成员，这些成员之前被您传递的参数遮蔽。

Answer 2

“谁能提供有关如何最好地修改我的代码以使 int *coeff 和 int order 成为私有成员的指导？”

您在main()中的代码

P1.display(P1.coeff, P1.order);

现在仍然尝试直接访问这些private 类成员，这当然行不通（这就是我们将这些成员设为private 的原因）。

对于您的情况，您根本不需要将这些变量作为参数传递给该函数，因为实际值已经可用于 class Poly 的 P1 实例。

将您的成员函数签名更改为

int display();

和定义到

int Poly::display() {
    int i;
    int j;
    for (i = order; i >= 0; i--) {
        cout << coeff[i] << "x^" << i;
        if ((i - 1) != -1) {
            cout << "+";
        }
    }
    cout << "\n";
    return 0;
}

请注意，在Poly::display() 成员函数中访问order 或coeff 可以被视为等效于this->order 和this->coeff。

类成员函数可以认为是隐含了this指针，可以直接访问（private）类成员变量，不需要指定这些为参数。

按照上述建议更改内容后，您可以调用

P1.display();

将其他签名/访问权限更改为coeff、order 类似。

---

注意：

您的类声明中存在更多缺陷。像

void addition(Poly P1, Poly P2);

效果不好。您希望将实际实例作为左侧值进行操作，而将另一个作为常量右侧值进行操作。恕我直言，您的成员函数应如下所示

Poly& addition(const Poly& rhs) {
     // perform addition operations with this and rhs
     return *this;
}

Poly addition(const Poly& rhs) const {
     Poly result = *this;
     // perform addition operations with result and rhs
     return result;
}