32 继承中的访问级别

目录

• 1 问题
• 2 继承中的访问级别

1 问题

• 子类是否可以直接访问父类的私有成员？

  • 根据面对对象理论：子类拥有父类的一切属性和行为 => 子类能够直接访问父类的私有成员
  • 根据 C++ 语法：外界（子类也是父类的外界）不能直接访问类的 private 成员 => 子类不能直接访问父类的私有成员

• 示例：继承中的访问级别

  • Dmeo

    #include <iostream>
    #include <string>
    
    using namespace std;
    
    class Parent
    {
    private:
        int mv;
    public:
        Parent()
        {
            mv = 100;
        }
        
        int value()
        {
            return mv;
        }
    };
    
    class Child : public Parent
    {
    public:
        int addValue(int v)
        {
            mv = mv + v;    // ???? 如何访问父类的非公有成员
        }
    };
    
    int main()
    {   
        return 0;
    }
    

  • 编译

    test.cpp: In member function ‘int Child::addValue(int)’:
    test.cpp:8:13: error: ‘int Parent::mv’ is private
             int mv;
                 ^
    test.cpp:23:13: error: within this context
                 mv = mv + v;
    

2 继承中的访问级别

• 面对对象中的访问级别不只是 public 和 private

• 可以定义 protected 访问级别

• 关键字 protected 的意义

  • 修饰的成员不能被外界直接访问
  • 修饰的成员可以被子类直接访问

• 示例：protected 使用

  • Demo

    #include <iostream>
    
    using namespace std;
    
    class Parent
    {
    protected:
        int mv;
    public:
        Parent()
        {
            mv = 100;
        }
        
        int value()
        {
            return mv;
        }
    };
    
    class Child : public Parent
    {
    public:
        int addValue(int v)
        {
            mv = mv + v;    
        }
    };
    
    int main()
    {   
        Parent p;
        
        cout << "p.mv = " << p.value() << endl;
        
        p.mv = 1000;    // error：外界无法访问
        
        Child c;
        
        cout << "c.mv = " << c.value() << endl;
        
        c.addValue(50);
        
        cout << "c.mv = " << c.value() << endl;
        
        c.mv = 10000;  // error：外界无法访问
        
        return 0;
    }
    

  • 编译运行

    test.cpp: In function ‘int main()’:
    test.cpp:8:13: error: ‘int Parent::mv’ is protected
             int mv;
                 ^
    test.cpp:33:7: error: within this context
         p.mv = 1000;
           ^
    test.cpp:8:13: error: ‘int Parent::mv’ is protected
             int mv;
                 ^
    test.cpp:41:7: error: within this context
         c.mv = 1000;
           ^
    

• 问题：为什么面对对象中需要 protected ？

  • protected 关键字是为了继承而专门设计的
  • 没有 protected 就无法完成真正意义上的代码复用

• 示例：组合和继承的综合实例

  • Dmeo

    #include <iostream>
    #include <string>
    #include <sstream>
    
    using namespace std;
    
    class Object
    {
    protected:
        string mName;
        string mInfo;
    public:
        Object()
        {
            mName = "Object";
            mInfo = "";
        }
        string name()
        {
            return mName;
        }
        string info()
        {
            return mInfo;
        }
    };
    
    class Point : public Object
    {
    private:
        int mX;
        int mY;
    public:
        Point(int x = 0, int y = 0)
        {
            ostringstream s;
            
            mX = x;
            mY = y;
            mName = "Point";
            
            s << "P(" << mX << ", " << mY << ")";
            
            mInfo = s.str();
        }
        int x()
        {
            return mX;
        }
        int y()
        {
            return mY;
        }
    };
    
    class Line : public Object
    {
    private:
        Point mP1;
        Point mP2;
    public:
        Line(Point p1, Point p2)
        {
            ostringstream s;
            
            mP1 = p1;
            mP2 = p2;
            mName = "Line";
            
            s << "Line from " << mP1.info() << " to " << mP2.info();
            
            mInfo = s.str();
        }
        Point begin()
        {
            return mP1;
        }
        Point end()
        {
            return mP2;
        }
    };
    
    int main()
    {   
        Object o;
        Point p(1, 2);
        Point pn(5, 6);
        Line l(p, pn);
        
        cout << o.name() << endl;
        cout << o.info() << endl;
        
        cout << endl;
        
        cout << p.name() << endl;
        cout << p.info() << endl;
        
        cout << endl;
        
        cout << l.name() << endl;
        cout << l.info() << endl;
        
        return 0;
    }
    

  • 编译运行