c++ - 从 C++ 中的类访问私有(private)变量

Question

我正在做一个项目，我想在一个类中声明私有(private)变量，因为我在很多地方读到它比将它们声明为公共(public)变量要好，但我如何在 main 中访问它们？我应该使用什么样的功能来使它们易于访问？我想通过一个不是像我所做的那样来自 main 的函数来解决系统问题。到目前为止，这就是我的代码，

#include <iostream>
#include <limits>

using namespace std;

class Equation
{
    private:
        int a1, a2, b1, b2, c1, c2;

    public:
};

int main() 
{
    int a, b, c, d, e, f;
    cout << "\n" << endl;
    cout << "                        **** Hello ****               \n\n" << endl;
    cout << "This is a program to solve a system of equation" << endl;
    cout << "Equations will look like a1*x+b1*y=c1" << endl;
    cout << "and  a2*x+b2*y=c2\n" << endl;
    cout << "Enter the values for the first equation \n" << endl;

    while ((cout << "Enter the value of a1 :\n") && !(cin >> a)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    while ((cout << "Enter the value of a2 :\n") && !(cin >> b)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    while ((cout << "Enter the value of b1 :\n") && !(cin >> c)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    while ((cout << "Enter the value of b2 :\n") && !(cin >> d)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    while ((cout << "Enter the value of c1 :\n") && !(cin >> e)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    while ((cout << "Enter the value of c2 :\n") && !(cin >> f)) 
    {
        cout << "Invalid input, please enter a number \n" << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    cout << "The first equation is : " << a << "x" <<
        "+" << c << "y" <<
        "=" << e << "\n" <<
        endl;

    cout << "The second equation is : " << b << "x" <<
        "+" << d << "y" <<
        "=" << f << "\n" <<
        endl;

    double x = ((c * e) - (b * f)) / ((a * e) - (b * d));
    double y = ((a * f) - (c * d)) / ((a * e) - (b * d));

    cout << "The solution of the system is " <<
        "x = " << x << "\t" <<
        "y = " << y <<
        "\n\n" <<endl;
    cout << "                        **** Thank You ****                 " << endl;

    return 0;
}

Answer 1

一个非常不受欢迎的选择是使用 setter 和 getter。

class A {
  public:
    void set_life(int life) { life_ = life; }
    void set_greeting(std::string greeting) { greeting_ = greeting; }
    int get_life() const { return life_; }
    std::string get_greeting() const { return greeting_; }
  private:
    int life_;
    std::string greeting_;
};

这些将以下列方式使用:

int main() {
  A a;
  a.set_life(42); // My object's life_ variable is now 42.
  a.set_greeting("Hello, World!"); // My object's greeting_ variable is now "Hello, World!".

  std::cout << a.get_greeting() << " The meaning of life is " << a.get_life() << '\n';
  // "Hello World! The meaning of life is 42
}

这被认为是一种失礼，因为如果您将 getter 和 setter 作为每个私有(private)变量的规则引入，您还不如将变量公开 - 它们无论如何都可以更改。

更好的做法是使用您的构造函数来初始设置您的变量，并创建一个方法来使用您的变量执行您需要的操作 - 程序员不需要了解它们(因此是私有(private)的)。

class A {
  public:
    // Construct life_ with life and greeting_ with greeting
    A(int life, std::string greeting) : life_(life), greeting_(greeting) {}
    void print_message() const { 
        std::cout << greeting_ << " The meaning of life is " << life_ << '\n';
    }
  private:
    int life_;
    std::string greeting_;
};   

现在你的 main 看起来像这样:

int main() {
  A a(42, "Hello, World!");
  a.print_message();
}