Access subclass data member from superclass array initialized using subclass objects

Question

A minimal reproducible example:

#include<iostream>

class Super
{
    private:
    int data1;
    
    public:
    virtual int getData1(){return data1;}
};

class Sub1:public Super
{
    private:
    static const int data1 = 1;
} sub1;

class Sub2:public Super
{
    private:
    static const int data1 = 3;
} sub2;

int main()
{
    Super obj[]={sub1, sub2};
    std::cout<<obj[0].getData1()<<'\n';
    std::cout<<obj[1].getData1()<<'\n';
    return 0;
}

What I need:
1
3

What I got:
0
0

How can I achieve this?

I found the following during my search -
Are static variables in a base class shared by all derived classes?
Accessing subclass members from a superclass pointer C++
C++ override a member variable

Accessing subclass members from a superclass pointer C++ looks similar but it doesn't solve my query.

EDIT:
I was informed that this happens due to object slicing.

See also: What is slicing. Even if you override getData1 in Sub1 and Sub2, that won't do you any good if you have an array of Super objects. — Nathan Pierson
– Nathan Pierson, Commented Apr 1, 2021 at 6:25

prehistoricpenguin · Accepted Answer · 2021-04-01 06:53:48Z

1

You may reference this similar question: What is object slicing?

The ways to avoid this is to use pointers or references, we may consider a vector of unique_ptr or boost::ptr_vector or vector of reference_wrapper, but to use reference we need to be careful since we may encounter dangling reference then get a crash(The referenced object may be destroyed).

#include <boost/ptr_container/ptr_vector.hpp>
#include <iostream>
#include <memory>

class Super {
 private:
  int data1;

 public:
  virtual int getData1() { return data1; }
};

class Sub1 : public Super {
 private:
  static const int data1 = 1;

 public:
  int getData1() { return data1; }
} sub1;

class Sub2 : public Super {
 private:
  static const int data1 = 3;

 public:
  int getData1() { return data1; }
} sub2;

void choice1() {
  std::unique_ptr<Super> obj[] = {std::make_unique<Sub1>(),
                                  std::make_unique<Sub2>()};
  std::cout << obj[0]->getData1() << '\n';
  std::cout << obj[1]->getData1() << '\n';
}
void choice2() {
  boost::ptr_vector<Super> vec;
  vec.push_back(new Sub1);
  vec.push_back(new Sub2);
  std::cout << vec[0].getData1() << '\n';
  std::cout << vec[1].getData1() << '\n';
}

edited Apr 1, 2021 at 6:53

answered Apr 1, 2021 at 6:48

prehistoricpenguin

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3 Comments

Kitswas Over a year ago

Does not compile. fatal error: boost/ptr_container/ptr_vector.hpp: No such file or directory Executes well without #include <boost/ptr_container/ptr_vector.hpp> and choice2()

prehistoricpenguin Over a year ago

You need to install boost library. If your are using ubuntu/debian, you can simply type sudo apt install libboost-dev; For other Linux or Windows platform, just google it.

Kitswas Over a year ago

Thanks for the answer. However, your choice1() method doesn't work if the functions are not overridden. Is there a way to avoid that?

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