Call through the base class abstract method [duplicate] - c++

Question

This question already has answers here:
C++ virtual method not called as desired
(4 answers)
Closed 5 years ago.
I have an interface from which a user derives multiple classes which I have no knowledge of but still I want to call these derived classes common method Run().
The Event class is intended to be an interface so I know how to call my unknown UserEvents derived class, since they all must have that Run() method implemented.
I currently have some code and get an error that CallEvent can't allocate an abstract Event. I understand the error, but don't know how I can go about and execute this correctly.
Here's some minimal code example (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);
}

Answer 1

I took your sample code (Like so) and tried to make it running (for illustration):
#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;
}
Now, it is compilable and runnable. Output:
Hello World!
in CallEvent(Event *WhatEvent):
Derived Event Dispatched 123
in CallEvent(Event &WhatEvent):
Derived Event Dispatched 123
(Life demo on ideone)
I annotated every modification in comments inside the sample code.