条款39：明智而审慎地使用private继承

阻止派生类重新定义虚函数, 在java和C#中可以很简单的实现(finale&sealed),

但在C++中则需要使用public继承+复合(composition)的方法;

注意: 使用私有继承(private), 无法实现此要求.

如:

/************************************************* 
File: test.cpp 
Copyright: C.L.Wang 
Author: C.L.Wang 
Date: 2014-04-11 
Description: test 
Email: morndragon@126.com 
**************************************************/
      
/*eclipse cdt, gcc 4.8.1*/
      
#include <iostream>  
      
using namespace std;  
      
class Timer {  
public:  
    virtual void onTick() const {  
        std::cout << "Tick! Tick! Tick! " << std::endl;  
    }  
};  
      
class Widget : private Timer {  
public:  
    void say() {  
        onTick();  
    }  
private:  
    virtual void onTick() const override{  
        std::cout << "Widget Tick!" << std::endl;  
    }  
};  
      
class AnotherWidget : private Widget {  
public:  
    void say() {  
        onTick();  
    }  
private:  
    virtual void onTick() const override{  
        std::cout << "Another Widget Tick!" << std::endl;  
    }  
};  
      
int main () {  
    Widget w;  
    w.say();  
    AnotherWidget aw;  
    aw.say();  
    return 0;  
}

输出:

Widget Tick!  
Another Widget Tick!

注意: 私有继承, 仍然可以继续重新定义虚函数;

 

 

具体实现:public继承+复合的方式, 代码实现, 如下:

/************************************************* 
File: test.cpp 
Copyright: C.L.Wang 
Author: C.L.Wang 
Date: 2014-04-11 
Description: test 
Email: morndragon@126.com 
**************************************************/
      
/*eclipse cdt, gcc 4.8.1*/
      
#include <iostream>  
      
using namespace std;  
      
class Timer {  
public:  
    virtual void onTick() const {  
        std::cout << "Tick! Tick! Tick! " << std::endl;  
    }  
};  
      
class Widget : private Timer {  
public:  
    void say() {  
        timer.onTick();  
    }  
private:  
    class WidgetTimer : public Timer {  
    public:  
        virtual void onTick() const {  
            std::cout << "Widget Tick! " << std::endl;  
        }  
    };  
    WidgetTimer timer;  
};  
      
int main () {  
    Widget w;  
    w.say();  
    return 0;  
}

输出:

Widget Tick!