Question

Class DVD inherits class Media, has one more variable than the base class does.

I declare a pointer:

Media* ptr = new DVD(...);

I want to print out the content of the DVD, so the following code works as expected:

ptr->print(cout);

But using the overloaded << operator only invokes the base class print() function:

cout << *ptr << endl;

So it prints out only the ID, not with the name of the director.

One way to solve this problem is to modify the overload << operator a bit to make it accept pointers, so:

cout << ptr << endl;

should work, but somehow I must find the way to make cout << *ptr << endl; work as expected.

Any advice?

The thing is I can’t make the base class (Media) abstract because I need to call an instance of it in the overloading ostream operator, so a pointer of the base class can not invoke the overloading function of the derived class, which it’s pointing to.

Code:

#include <iostream>
using namespace std;

class Media{
    private:
        int id;
    public:
        Media(int _id) : id(_id) {}
        virtual ~Media();
        virtual void print(ostream &out);
        friend ostream& operator << (ostream& out, Media aMedia);
};
Media::~Media(){}

class DVD : public Media {
    private:
        string director;
    public:
        DVD(int _id, string _director = "unknown") : Media(_id), director(_director) {}
        ~DVD();
        void print(ostream &out);
};
DVD::~DVD(){}

void Media::print(ostream& out){
    out << "ID " << id;
}
void DVD::print(ostream& out){
    out << "DVD: ";
    Media::print(out);
    out << " Directed by " << director;
}
ostream& operator << (ostream& out, Media aMedia){
    aMedia.print(out);
    return out;
}

int main() { 
    Media *ptr = new DVD(352, "Stephen Spielberg"); 
    ptr->print(cout); // Prints out: "DVD: ID 352 Directed by Stephen Spielberg". Correct!
    cout << endl; 
    cout << *ptr << endl; //Prints out: "ID 352" Incorrect!
} 

Answer 1

Problem is in this declaration ostream& operator << (ostream& out, Media aMedia). You are accepting the parameter aMedia by copy which causes object slicing, accept it by using a reference by changing the signature to ostream& operator << (ostream& out, const Media& aMedia) .

Because of the slice when you do cout << *ptr , a copy of the DVD is created of type Media (i.e. DVD is sliced to a Media), now when you call print since the type of the object is Media call goes to the Media::print. You can read more about object slicing here.