如何使一个类的公共成员私有？-Java 学习之路

我是编程（一般）和C（特别是）的新手 . 我正在尝试使用以下公共成员变量并将其设为私有：

int *coeff;
int order;

不幸的是，我看到以下错误：

'Poly :: coeff'：无法访问'Poly'类中声明的私有成员

和

'Poly :: order'：无法访问'Poly'类中声明的私有成员

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;
}

任何人都可以提供有关如何最好地修改我的代码的指导，以便int * coeff和int order是私有成员吗？

首先十分感谢 . -Ryan

2 回答

2
制作东西 private 的想法意味着你不允许访问成员之外或 friend 函数（这是 private 的目的） . main 之类的代码在 main 中失败，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） .

我建议你改变 Poly::display(int *coeff, int order) 的定义，不要求你传入 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";
}
```
在该成员函数内部，变量 order 和 coeff 隐式地拾取您之前被传递的参数所遮蔽的类成员 .
回复于 2024-05-15T13:38:55+08:00
3
“任何人都可以提供有关如何最好地修改我的代码的指导，以便int * coeff和int order是私有成员吗？”

你的代码在 main()
```
P1.display(P1.coeff, P1.order);
```
仍然试图直接访问这些现在 private 类成员，这赢得了't work of course (that'为什么我们让这些成员 private ） .

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
```
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，访问 Poly::display() 成员函数中的 order 或 coeff 可以读取为等效于 this->order 和 this->coeff .

可以认为类成员函数隐式具有 this 指针，并且可以直接访问（ private ）类成员变量，而无需将这些指定为参数 .

在改变上面提出的东西后，你可以打电话
```
P1.display();
```
更改其他签名/访问 coeff ， order 类似 .

NOTE:

您的类声明中存在更多缺陷 . 就像是
```
void addition(Poly P1, Poly P2);
```
不会很好 . 您希望将实际实例作为左手值进行操作，将另一个作为常量右手值进行操作 . 恕我直言，你的会员功能应该是这样的
```
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;
}
```
回复于 2024-05-15T13:38:55+08:00

如何使一个类的公共成员私有？

2 回答

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