Calling virtual function from most derived class

Question

I have a list of objects of type A, that may contain instances of any number of classes derived from A. I am calling a function on each member of the list, and wish to have the function from the most derived class be called. However, the function of the base class is called. How can I get it to be the case that the most derived class is used?

The following code illustrates my problem. The code outputs "In a" but I want it to output "In b".

#include <iostream>

class A {
public:
    virtual void func()
    {
        std::cout << "In A" << std::endl;
    }
};

class B : public A {
public:
    virtual void func()
    {
        std::cout << "In B" << std::endl;
    }
};

int main()
{
    A a = B();
    a.func();
    return 0;
}

score 6 · Accepted Answer · edited May 23 '17 at 10:28

6

When you do

A a = B();

you are slicing the object. What you have is only an instance of an A object.

To make polymorphism work you need to either use references:

B b;
A& a = b;

or use pointers

A* a = new B;

edited May 23 '17 at 10:28

Community

1
1

answered Apr 12 '16 at 12:55

Some programmer dude

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Thanks Joachim, that solved my problem – JHobern Apr 12 '16 at 12:57
Or use smart pointers! std::unique_ptr and std::shared_ptr – Ceros Apr 12 '16 at 13:02
@Ceros Those are just a variant of pointers in general. And I also won't recommend using the new smart pointers a simple auto-deleting pointers, but as part of an *ownership* model. – Some programmer dude Apr 12 '16 at 13:18

Calling virtual function from most derived class

1 Answers1