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C++ virtual method not called as desired

(4个答案)


2年前关闭。




我有一个接口(interface),用户可以从该接口(interface)派生我不知道的多个类,但我仍然想将这些派生类称为通用方法Run().Event类旨在用作接口(interface),因此我知道如何调用我的未知 UserEvents派生类,因为它们都必须实现了Run()方法。

我目前有一些代码,并得到一个错误CallEvent can't allocate an abstract Event。我了解错误,但不知道如何解决并正确执行此操作。

这是一些最小的代码示例(WandBox):
#include <iostream>

class Event
 {
 public:
      virtual void Run(int Param) = 0;
 };

 // This is a user event and I have no idea what the class name is,
 // but I still have to call it's method Run() that is common to the interface "Event"
 class UserEvent : public Event
 {
 public:
      virtual void Run(int Param) { std::cout << "Derived Event Dispatched " << Param << std::endl;};
 };

 // This parameter is of pure abstract base class Event because
 // I have no idea what my user class is called.
 void CallEvent(Event WhatEvent)
 {
      WhatEvent.Run(123);
 };

 int main()
 {
      std::cout << "Hello World!" << std::endl;
      UserEvent mE;
      CallEvent(mE);
 }

最佳答案

我获取了您的示例代码(Like so),并尝试使其运行(用于说明):

#include <iostream>

class Event {
  public:
    virtual void Run(int Param) = 0;
};

// This is a user event and I have no idea what the class name is,
// but I still have to call it's method Run() that is common to the interface "Event"
class UserEvent: public Event {
  public:
    virtual void Run(int Param)
    {
      std::cout << "Derived Event Dispatched " << Param << std::endl;
    }
};

// This parameter is of pure abstract base class Event because
// I have no idea what my user class is called.
void CallEvent(Event *WhatEvent)
{
  std::cout << "in CallEvent(Event *WhatEvent):" << std::endl;
  // Huh? WhatEvent = new Event();
  // wrong: WhatEvent.Run(123);
  // Instead, use ->.
  // For pointers, check for non-nullptr is very reasonable:
  WhatEvent->Run(123);
  // obsolete: delete WhatEvent;
}

// second approach using a reference (as recommended in comments):
void CallEvent(Event &WhatEvent)
{
  std::cout << "in CallEvent(Event &WhatEvent):" << std::endl;
  WhatEvent.Run(123); // for references - select operator . is fine
}

int main()
{
  std::cout << "Hello World!" << std::endl;
  /* nullptr does not make sense:
   * UserEvent *mE = nullptr;
   * Go back to original approach:
   */
  UserEvent mE;
  CallEvent(&mE); // calling the first (with Event*)
  CallEvent(mE); // calling the second (with Event&)
  return 0;
}

现在,它是可编译且可运行的。输出:

Hello World!
in CallEvent(Event *WhatEvent):
Derived Event Dispatched 123
in CallEvent(Event &WhatEvent):
Derived Event Dispatched 123

(ideone上的生活演示)

我在示例代码中的注释中注释了每个修改。

07-24 09:46
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