[C++] Interface

#include <iostream>


using namespace std;

class Printable {
	friend ostream& operator<<(ostream& os, const Printable& obj);
public:
	virtual void print(ostream& os) const = 0;
	virtual ~Printable() {}
};

ostream& operator<<(ostream& os, const Printable& obj) {
	obj.print(os);
	return os;
}

class class1 : public Printable {
public:
	virtual void print(ostream& os) const override {
		os << "class1" << endl;
	}
};

class class2 : public Printable {
public:
	virtual void print(ostream& os) const override {
		os << "class2" << endl;
	}
};

int main() {
	class1 cls1;
	class2 cls2;
	cout << cls1 << endl;
	cout << cls2 << endl;
}