【C++】 44继承中的访问级别 protected关键字

子类是否可以直接访问父类的私有成员吗？

编程实验：
继承中的访问级别44-1.cpp

#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;
}

继承中的访问级别

面向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

44-2.cpp

#include <iostream>
#include <string>

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;    // ???? 如何访问父类的非公有成员
    }
};

int main()
{   
    Parent p;
    cout << "p.mv = "<<p.value() << endl;
  //  p.mv = 100;    error
    Child c;
    cout << "c.mv = "<<c.value() << endl;
    c.addValue(50);
    cout << "c.mv = "<<c.value() << endl;
 //   c.mv = 10000;    error
    return 0;
}

输出：

p.mv = 100
c.mv = 100
c.mv = 150

为什么面向对象中需要protected？

#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;
}

输出结果：

Object


Point
P(1, 2)

Line
Line from P(1, 2) to P(5, 6)