Member values not accessed in vector of instances of different subclasses - c++

Question

Jumping off of the above question Making a vector of instances of different subclasses : when implementing a vector of (pointers to) different subclasses (initialized as vector<Base*> objects), I expect to be able to access the correct member variables based on the subclass called.
Below is sample code:
#include <iostream>
#include <vector>
#include <memory>
using namespace std;
class Entity {
public:
int index;
Entity() {};
virtual ~Entity() {};
virtual void hit() = 0;
};
class Mesh : public Entity {
public:
int index;
Mesh(int x) {this->index=x;};
virtual void hit() {}
};
int main() {
vector<unique_ptr<Entity>> objects;
objects.push_back(unique_ptr<Entity>(new Mesh(35)));
objects.push_back(unique_ptr<Entity>(new Mesh(10)));
for ( int i = 0 ; i < objects.size() ; i++ )
cout << objects[i]->index << endl;
return 0;
}
Where I expect
35
10
to be printed, meanwhile I get instead.
0
0
How can I access the correct member variable values in this scenario?

Answer 1

The problem is due to a misunderstanding on inheritance. If you redefine your Mesh as follows, it would work as expected:
class Mesh : public Entity {
public:
//int index; // No don't redefine: here you'd have two different indexes
Mesh(int x) {this->index=x;};
void hit() override {} // extraa safety: use override instead of virtual to be sure to override
};
Online demo
Not related to the problem, but some more thoughts:
You can of course only use the interface of the base class (here Entity).
It is not very prudent to expose the index member publicly. A safer option would be to have it private, and access it via a public getter. If the index should be set only at construction, the Entity constructor should take care of it, otherwise you could consider a public setter.