C++保护成员

#include <iostream>

using namespace std;

class Box {
   protected:
      double width;
};

class SmallBox:Box {    // SmallBox is the derived class.
   public:
      void setSmallWidth( double wid );
      double getSmallWidth( void );
};

// Member functions of child class
double SmallBox::getSmallWidth(void) {
   return width ;
}

void SmallBox::setSmallWidth( double wid ) {
   width = wid;
}

// Main function for the program
int main() {
   SmallBox box;

   // set box width using member function
   box.setSmallWidth(5.0);
   cout << Width of box : << box.getSmallWidth() << endl;

   return 0;
}