一个类的私有数据成员能在其派生类中直接使用吗？
下面这段代码来自谭浩强老师的《C++程序设计》一书399页，有一处不懂，请大家帮忙看一下
#include<iostream>using namespace std;//声明类Pointclass Point{public:Point(float x=0,float y=0);void setPoint(float,float);float getX() const{return x;}float getY() const{return y;}friend ostream & operator<<(ostream &,const Point &);protected: float x,y;};//下面定义Point类的成员函数//Point的构造函数Point::Point(float a,float b){ x=a; y=b;}//设置x和y的坐标值void Point::setPoint(float a,float b){ x=a; y=b;}//重载运算符“<<”，使之能输出点的坐标ostream & operator<<(ostream &output,const Point &p){output<<"["<<p.x<<","<<p.y<<"]"<<endl;return output;}//声明Point类的派生类Circleclass Circle:public Point{public:Circle(float x=0,float y=0,float r=0);void setRadius(float);float getRadius() const;float area() const;friend ostream &operator<<(ostream &,const Circle &);//重载运算符“<<”private: float radius;};//定义构造函数，对圆心坐标和半径初始化Circle::Circle(float a,float b,float r):Point(a,b),radius(r){}//设置半径值void Circle::setRadius(float r){radius=r;}//读取半径值float Circle::getRadius() const {return radius;}//计算圆面积float Circle::area() const{return (3.14159f)*radius*radius;}//重载运算符“<<”，使之输出圆的信息ostream &operator<<(ostream &output,const Circle &c){ output<<"center=["<<c.x<<","<<c.y<<"],"<<"radius="<<c.radius<<","<<"area="<<c.area()<<endl; return output;}//声明Circle类的派生类Cylinderclass Cylinder:public Circle{public: Cylinder(float x=0,float y=0,float r=0,float h=0); void setHeight(float); float getHeight() const; float area() const; float volume() const; friend ostream &operator<<(ostream &,const Cylinder &);protected: float height;};//定义构造函数Cylinder::Cylinder(float a, float b, float r, float h):Circle(a,b,r),height(h){}//设置圆柱高度void Cylinder::setHeight(float h){height=h;}//读取圆柱高度float Cylinder::getHeight() const{return height;}//计算圆表面积float Cylinder::area() const{return 2*Circle::area()+2*(3.14159f)*radius*height;}[color=#FF0000]这里的radius是Circle的私有成员，能直接这样用吗？[/color]//计算圆柱体积float Cylinder::volume() const{return Circle::area()*height;}//重载运算符“<<”ostream &operator<<(ostream &output,const Cylinder &cy){output<<"center=["<<cy.x<<","<<cy.y<<"],r="<<cy.radius<<",h="<<cy.height<<"\narea="<<cy.area()<<",volume="<<cy.volume()<<endl;[color=#FF0000]这里的radius是Circle的私有成员，能直接这样用吗？[/color]return output;}int main(){Cylinder cy1(3.5f,6.4f,5.2f,10);cout<<"\noriginal cylinder:\nx="<<cy1.getX()<<",y="<<cy1.getY()<<",r="<<cy1.getRadius()<<",h="<<cy1.getHeight()<<",area="<<cy1.area()<<"\n"<<",volume="<<cy1.volume()<<endl;cy1.setHeight(15);cy1.setRadius(7.5f);cy1.setPoint(5,5);cout<<"\nnew cylinder:\n"<<cy1;Point &pRef=cy1;cout<<"\npRef as a point:\n"<<pRef;Circle &cRef=cy1;cout<<"\ncRef as a circle:\n"<<cRef;return 0;}
1>e:\software learning\general software\softwarepractise\c++practise\ex12_1\ex12_1\ex12_1.cpp(88) : error C2248: “Circle::radius”: 无法访问 private 成员(在“Circle”类中声明)
1> e:\software learning\general software\softwarepractise\c++practise\ex12_1\ex12_1\ex12_1.cpp(42) : 参见“Circle::radius”的声明
1> e:\software learning\general software\softwarepractise\c++practise\ex12_1\ex12_1\ex12_1.cpp(34) : 参见“Circle”的声明
1>生成日志保存在“file://e:\SOFTWARE LEARNING\GENERAL SOFTWARE\softwarepractise\c++practise\ex12_1\ex12_1\Debug\BuildLog.htm”
1>ex12_1 - 2 个错误，0 个警告
但在书上说可以编译通过.这本书已经出版这么多年，应该不会有错，应该是我错了。但实在找不到错在哪里？请大家帮忙给看一下。谢谢。
[解决办法]
不能。 。
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回答lz标题问题: 不能
建议： 不要看谭浩强的书。
[解决办法]
论坛里面回答这个问题必说： C++ primer
嫌厚可以选薄点的： Accelerated C++