How to invoke the overloaded operator in a derived classes, rather than the base, when pointed to as base in a collection

Question

As the title suggests, I am trying to invoke overloaded operators in derived classes, while all of the objects are in a collection of type "pointer to base". In this case, std::array<A*, #>. I think that the code examples will make my situation more clear.

classes.h:

#include <iostream>
        
class A
{
    friend std::ostream& operator<<(std::ostream& os,const A& a){
        os << "I am a A!";
        return os;
    }
};
        
class B : public A{
    friend std::ostream& operator<<(std::ostream& os,const B& b){
        os << "I am a B!";
        return os;
    }
};
        
class C: public  A{
    friend std::ostream& operator<<(std::ostream& os,const C& c){
        os << "I am a C!";
        return os;
    }
};

main.cpp:

#include <iostream>
#include <array>
#include "classes.h"

int main(){
    std::array<A*, 2> arr;

    arr[0] = new B;
    arr[1] = new C;

    std::cout << *arr[0] << "\n";
    std::cout << *arr[1] << "\n";

    return 0;
}

The output of this program is:

I am a A!
I am a A!

But I would like it to be:

I am a B!
I am a C!

Is there an elegant way of doing this?

In this particular case you might want to follow [this suggestion](https://stackoverflow.com/questions/11905648/overloading-with-inhertiance-and-polymorphism) and have a single `operator<<` that takes a `const A&` and invokes a virtual helper method — Nathan Pierson, Nov 16 '21 at 18:42
TL;DR of that dupe: make a polymorphic `print()` or `toString()` function that you call in the `operator<<()`. — scohe001, Nov 16 '21 at 18:43

score 2 · Answer 1 · edited Nov 16 '21 at 18:46

2

Declare a virtual function in the base class, like for example:

virtual std::ostream & out( std::ostream &os ) const;

The function may be protected.

And then override it in derived classes.

Then, in the friend operator<<, call the function.

edited Nov 16 '21 at 18:46

Remy Lebeau

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answered Nov 16 '21 at 18:44

Vlad from Moscow

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Hmm this answer [looks familiar](https://stackoverflow.com/a/40506381/2602718) – scohe001 Nov 16 '21 at 18:45
@scohe001 It is funny. It was my answer.:) – Vlad from Moscow Nov 16 '21 at 18:46
5 years later I think we can forgive you forgetting you'd answered it already. – user4581301 Nov 16 '21 at 18:47

How to invoke the overloaded operator in a derived classes, rather than the base, when pointed to as base in a collection

1 Answers1