Let's say I have a base class Animal from which a class Cow inherits, and a Barn class containing an Animal vector, and let's say the Animal class has a virtual function scream(), which Cow overrides.

With the following code:

Animal.h

#ifndef _ANIMAL_H
#define _ANIMAL_H
#include <iostream>
using namespace std;

class Animal {
public:
    Animal() {};
    virtual void scream() {cout << "aaaAAAAAAAAAAGHHHHHHHHHH!!! ahhh..." << endl;}
};

#endif  /* _ANIMAL_H */

Cow.h

#ifndef _COW_H
#define _COW_H

#include "Animal.h"

class Cow: public Animal {
public:
    Cow() {}
    void scream() {cout << "MOOooooOOOOOOOO!!!" << endl;}
};

#endif  /* _COW_H */

Barn.h

#ifndef _BARN_H
#define _BARN_H

#include "Animal.h"
#include <vector>

class Barn {
    std::vector<Animal> animals;

public:
    Barn() {}
    void insertAnimal(Animal animal) {animals.push_back(animal);}
    void tortureAnimals() {
        for(int a = 0; a < animals.size(); a++)
            animals[a].scream();
    }
};

#endif  /* _BARN_H */

and finally main.cpp

#include <stdlib.h>
#include "Barn.h"
#include "Cow.h"
#include "Chicken.h"

/*
 * 
 */
int main(int argc, char** argv) {
    Barn barn;
    barn.insertAnimal(Cow());
    barn.tortureAnimals();
    return (EXIT_SUCCESS);
}

I get this output:

aaaAAAAAAAAAAGHHHHHHHHHH!!! ahhh...

How should I code this to get MOOooooOOOOOOOO!!! (and whatever other classes inheriting Animal wants scream() to be) instead?