运算符<<和继承 [英] Operator << and inheritance

问题描述

我在C ++中具有以下类:

I have the following classes in C++:

class Event {
        //...
        friend ofstream& operator<<(ofstream& ofs, Event& e);

};


class SSHDFailureEvent: public Event {
    //...
    friend ofstream& operator<<(ofstream& ofs, SSHDFailureEvent& e);
};

我要执行的代码是:

main(){

 Event *e = new SSHDFailureEvent();
 ofstream ofs("file");
 ofs << *e; 

}

这是一种简化，但是我要做的是将几种类型的事件写入文件在一个文件中.但是，不是使用运算符<<在SSHDFailureEvent中，它使用运算符<<事件.有什么办法可以避免这种行为?

This is a simplification, but what I want to do is write into a file several type of Events in a file. However, instead of using the operator << of SSHDFailureEvent, it uses the operator << of Event. Is there any way to avoid this behavior?

谢谢

推荐答案

那是行不通的，因为它将为基类调用 operator<< .

That would not work, as that would call operator<< for the base class.

您可以在基类中定义虚拟函数 print ，然后重新定义所有派生类，并只将 operator<< 定义一次，

You can define a virtual function print in base class and re-define it all derived class, and define operator<< only once as,

class Event {

      virtual ofstream& print(ofstream & ofs) = 0 ; //pure virtual  

      friend ofstream& operator<<(ofstream& ofs, Event& e);
};

//define only once - no definition for derived classes!
ofstream& operator<<(ofstream& ofs, Event& e)
{
   return e.print(ofs); //call the virtual function whose job is printing!
}

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