Derived类如何从Base类继承一个静态函数？ [英] How can Derived class inherit a static function from Base class?

问题描述

  struct TimerEvent 
 {
 event Event; 
 timeval TimeOut; 
 static void HandleTimer（int Fd，short Event，void * Arg）; 
}; 
 

HandleTimer需要是静态的，因为我将它传递给C库（libevent）。

我想继承这个类。

感谢。

解决方案

容易继承该类：

  class Derived：public TimerEvent {
 ... 
};但是，你不能覆盖你的子类中的HandleTimer，并期望这个工作：
 
 
 
 
  TimerEvent * e = new Derived（）; 
 e-> HandleTimer（）; 
  
这是因为静态方法在vtable中没有条目，虚拟。然而，你可以使用void * Arg来传递一个指向你的实例的指针...：
  struct TimerEvent { 
 virtual void handle（int fd，short event）= 0; 
 
 static void HandleTimer（int fd，short event，void * arg）{
（（TimerEvent *）arg） - > handle（fd，event）; 
} 
}; 
 
 class Derived：public TimerEvent {
 virtual void handle（int fd，short event）{
 // whatever 
} 
}; 
  
这样，HandleTimer仍然可以从C函数使用，只要确保始终通过 对象作为void * Arg。
 
struct TimerEvent
{
   event Event;
   timeval TimeOut;
   static void HandleTimer(int Fd, short Event, void *Arg);
};
HandleTimer needs to be static since I'm passing it to C library (libevent).

I want to inherit from this class. How can this be done?

Thanks.
解决方案
You can easily inherit from that class:
class Derived: public TimerEvent {
    ...
};
However, you can't override HandleTimer in your subclass and expect this to work:
TimerEvent *e = new Derived();
e->HandleTimer();
This is because static methods don't have an entry in the vtable, and can't thus be virtual. You can however use the "void* Arg" to pass a pointer to your instance... something like:
struct TimerEvent {
    virtual void handle(int fd, short event) = 0;

    static void HandleTimer(int fd, short event, void *arg) {
        ((TimerEvent *) arg)->handle(fd, event);
    }
};

class Derived: public TimerEvent {
    virtual void handle(int fd, short event) {
        // whatever
    }
};
This way, HandleTimer can still be used from C functions, just make sure to always pass the "real" object as the "void* Arg".

                        
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