从C ++中的类访问私有变量

Question

我在一个项目上工作，我想在一个类中声明私有变量，因为我在许多地方都读到它比声明它们​​为公开要好，但是如何在main中访问它们呢？我应该使用哪种功能使它们可访问？我想通过功能解决系统问题，而不是像我做的那样。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_;
};

这些将通过以下方式使用：

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
}

这被认为是虚假的，因为如果您将getter和setter作为每个私有变量的规则引入，则不妨将这些变量设为公用-可以随时对其进行更改。

更好的做法是使用构造函数来初始设置变量，并创建一种方法来对变量进行所需的操作-程序员无需了解变量（因此不公开）。

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_;
};   

现在您的主体看起来像这样：

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