Why doesn't Polymorphed function work? c++

Asked Jun 22 '22 at 14:14

Active Jun 23 '22 at 15:28

Viewed 36 times

I am currently preparing for an exam, by learning Polymorphism. I wrote this Programm:

#include <iostream>
#include <vector>
using namespace std;

class A {
public:
    int x = 3;

    virtual void print() { cout << 3 << endl; }
};

class B : public A {
public:
    int x = 5;

    void print() { cout << 5 << endl; }
};

int main() {
    B b;
    A a;
    vector<A> name1;

    name1.push_back(b);
    name1[0].print();
    cout<< name1[0].x << endl;

    return 0;
}

And expected an output of "5". Why does it output "3"? Shouldn't the virtual funcion in A "forward" the call to the subclass "B"?

EDIT: From the answers I got, and further research, this happened:

Cutting of the complete Part of Class B, becuase I copied only Class A
Trying to access the functions of the B Class therefore fails.

Fix:

Change the Code as following:
```
 vector<A*> name1;
```

and

name1.push_back(&b);

edited Jun 23 '22 at 15:28

asked Jun 22 '22 at 14:14

NoNToxic

Make the call using pointer or reference and you'll get your expected output. See [DEMO](https://onlinegdb.com/mIBN1XUuL). In the above linked demo, you'll get the output you expect. – Jason Jun 22 '22 at 14:17
2

Any object in a `std::vector` is going to be an `A`. – Drew Dormann Jun 22 '22 at 14:19
You have [Object Slicing](https://stackoverflow.com/questions/274626/what-is-object-slicing) – Jarod42 Jun 22 '22 at 14:46
`vector` only holds `A` objects. `name1.push_back(b)` takes the `A` portion of `b` and adds that *sliced* `A` to the vector. Polymorphism requires the use of *pointers* (including smart pointers) or *references*. – Eljay Jun 22 '22 at 15:52

Why doesn't Polymorphed function work? c++

0 Answers0