访问OBJ类的私人数据

在此处获取函数operator+ Overloading正在使用.运算符访问类的私人成员的代码。实际上我的问题是使用.运算符。用于直接访问私人会员？

   #include <iostream>
using namespace std;
class Box
{
   public:
      double getVolume(void)
      {
         return length * breadth * height;
      }
        void setLength( double len )
      {
          length = len;
      }
      void setBreadth( double bre )
      {
           breadth = bre;
      }
      void setHeight( double hei )
      {
      height = hei;
      }
                        // Overload + operator to add two Box objects.
  Box operator+(const Box& b)
      {
         Box box;
         box.length = this->length + b.length;     //These are my problem
         box.breadth = this->breadth + b.breadth;  //These are my problem
         box.height = this->height + b.height;     //These are my problem
         return box;
      }
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};
// Main function for the program
int main( )
{
   Box Box1;                // Declare Box1 of type Box
   Box Box2;                // Declare Box2 of type Box
   Box Box3;                // Declare Box3 of type Box
   double volume = 0.0;     // Store the volume of a box here
   // box 1 specification
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);
   // box 2 specification
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0);
   // volume of box 1
   volume = Box1.getVolume();
   cout << "Volume of Box1 : " << volume <<endl;
   // volume of box 2
    volume = Box2.getVolume();
    cout << "Volume of Box2 : " << volume <<endl;
   // Add two object as follows:
   Box3 = Box1 + Box2;
   // volume of box 3
   volume = Box3.getVolume();
   cout << "Volume of Box3 : " << volume <<endl;
   return 0;
    }

我不了解功能运营商如何使用.操作员来访问私人会员。我了解到只有成员函数才能访问类的私人成员。它们不是直接访问的，而不是该功能如何访问它？谢谢。