1 C++继承中的访问级别
1.1 protected访问级别
思考一下:子类是否可以直接访问父类的私有成员?
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。
protected关键字就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。
protected示例代码:
#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;
}
};
int main()
{
Parent p;
cout << "p.mv = " << p.value() << endl;
// p.mv = 1000; // error
Child c;
cout << "c.mv = " << c.value() << endl;
c.addValue(50);
cout << "c.mv = " << c.value() << endl;
// c.mv = 10000; // error
return 0;
}
1.2 定义类时访问级别的选择
定义类时访问级别的选择如下:
1.3 组合与继承的综合实例
#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(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;
}
参考资料: