C++ Advanced (7)-Inheritance and Derivation 3

Private inheritance and protected inheritance

Private inheritance (private) 

• Inherited access control

  • The public and protected members of the base class : both appear in the derived class as private ;

  • Private members of the base class: not directly accessible .

• access permission

  • Member functions in derived classes: you can directly access the public and protected members of the base class, but you cannot directly access the private members of the base class;

  • Objects through derived classes: no direct access to any members inherited from the base class.

Private inheritance example

#ifndef _POINT_H
#define _POINT_H
class Point {   //基类Point类的定义
public: //公有函数成员
    void initPoint(float x = 0, float y = 0) 
  { this->x = x; this->y = y;}
    void move(float offX, float offY) 
  { x += offX; y += offY; }
    float getX() const { return x; }
    float getY() const { return y; }
private:    //私有数据成员
    float x, y;
};  
#endif //_POINT_H

#ifndef _RECTANGLE_H
#define _RECTANGLE_H
#include "Point.h"
class Rectangle: private Point {    //派生类定义部分
public: //新增公有函数成员
    void initRectangle(float x, float y, float w, float h) {
        initPoint(x, y); //调用基类公有成员函数
        this->w = w;
        this->h = h;
    }
    void move(float offX, float offY) {   Point::move(offX, offY);  }
    float getX() const { return Point::getX(); }
    float getY() const { return Point::getY(); }
    float getH() const { return h; }
    float getW() const { return w; }
private:    //新增私有数据成员
    float w, h;
};
#endif //_RECTANGLE_H

#include <cmath>
using namespace std;

int main() {
    Rectangle rect; //定义Rectangle类的对象
    rect.initRectangle(2, 3, 20, 10);   //设置矩形的数据
    rect.move(3,2); //移动矩形位置
    cout << "The data of rect(x,y,w,h): " << endl;
    cout << rect.getX() <<", "  //输出矩形的特征参数
        << rect.getY() << ", "
        << rect.getW() << ", "
        << rect.getH() << endl;
    return 0;
}

Protected inheritance (protected)

• Inherited access control

  • The public and protected members of the base class : both appear in the derived class as protected ;

  • Private members of the base class : not directly accessible .

• access permission

  • Member functions in derived classes: you can directly access the public and protected members of the base class, but you cannot directly access the private members of the base class;

  • Objects through derived classes: no direct access to any members inherited from the base class.

• Features and functions of protected members

  • For the module that creates the object of its class, it has the same nature as the private member.

  • For its derived classes, it has the same properties as public members.

  • It not only realizes data hiding, but also facilitates inheritance and realizes code reuse.

  • If the derived class has multiple base classes, that is, multiple inheritance, each base class can be inherited in different ways.

Examples of protected members

class A {
protected:
    int x;
};

int main() {
    A a;
    a.x = 5;//错误
}
class A {
protected:
    int x;
};
class B: public A{
public:
    void function();
};
void B:function() {
    x = 5;   //正确
}

Examples of multiple inheritance

class A {
public:
    void setA(int);
    void showA() const;
private:
    int a;
};
class B {
public:
    void setB(int);
    void showB() const;
private:
    int b;
};
class C : public A, private B { 
public:
    void setC(int, int, int);
    void showC() const;
private:
    int c;
};

void  A::setA(int x) {
    a=x; 
}
void B::setB(int x) {
    b=x; 
}
void C::setC(int x, int y, int z) {
    //派生类成员直接访问基类的
    //公有成员
    setA(x); 
    setB(y); 
    c = z;
}

//其他函数实现略

int main() {
    C obj;
    obj.setA(5);
    obj.showA();
    obj.setC(6,7,9);
    obj.showC();
// obj.setB(6);  错误
// obj.showB(); 错误
    return 0;
}