子类是否可以直接访问父类的私有成员?
#include<iostream>
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;
}
};
int main()
{
return 0;
}
编译出错,那如何访问父类中的非公有成员呢?
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
#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; //编译会出错。虽然mv的访问级别是protected,但是在外部只有它的子类内部可以直接访问。
Child c;
cout << "c.mv = " << c.value() << endl;
c.addValue(50);
cout << "c.mv = " << c.value() << endl;
//c.mv = 200; //出错
return 0;
}
定义类时,访问级别的选择
继承与组合的综合实例:
#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()
{
mX = 0;
mY = 0;
}//这个地方也可以不使用这个无参构造函数,直接利用下面的Point(int x=0, int y=0),但是我不知道为什么?
Point(int x,int y)
{
ostringstream s;
mX = x;
mY = y;
s << "p(" << mX << "," << mY << ")" ;
mName = "Point";
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();
}
};
int main()
{
Object o;
cout << o.name() << endl;
cout << o.info() << endl;
cout << endl;
Point p(1,2);
Point pa(5,6);
cout << p.name() << endl;
cout << p.info() << endl;
Line l(p,pa);
cout << l.name() <<endl;
cout << l.info() <<endl;
return 0;
}
小结:
面向对象中的访问级别不只是public和private
protected修饰的成员不能被外界所访问
protected使得子类能够访问父类的成员
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