suggestion for https://www.reddit.com/r/cpp_questions/comments/2p93b3/how_do_i_inherit_a_overloaded_operator_in_c/

main.cpp

#include <iostream>
#include <string>

class vehicle {
public:
	vehicle(const std::string& model, const std::string& color, unsigned year, unsigned price):
		m_model{model}, m_color{color}, m_year{year}, m_price{price} {}

	// we have a virtual method, so we should make the dtor virtual too:
	virtual ~vehicle() = default;

	// get back the default-definitions of copy/move-ctor/assignment-operator:
	vehicle(const vehicle&) = default;
	vehicle(vehicle&&) = default;
	vehicle& operator=(const vehicle&) = default;
	vehicle& operator=(vehicle&&) = default;

	friend std::ostream& operator<<(std::ostream& stream, const vehicle& v) {
		v.print(stream);
		return stream;
	}

protected:
	virtual void print(std::ostream& stream) const {
		stream << 
			"year: " << m_year << "\n"
			"model: " << m_model << "\n"
			"color: " << m_color << "\n"
			"price: " << m_price << '\n';
	}

private:
	std::string m_model;
	std::string m_color;
	unsigned m_year;
	unsigned m_price;
};


class truck: public vehicle {
public:
	truck(const std::string& model, const std::string& color, unsigned year, unsigned price, unsigned passengers):
		vehicle{model, color, year, price}, m_passengers{passengers} {}

protected:
	void print(std::ostream& stream) const override {
		vehicle::print(stream);
		stream << "passengers: " << m_passengers << '\n';
	}
private:
	unsigned m_passengers;
};



int main() {
	const auto vehicle_1 = vehicle{"car", "black", 2013, 20'000};
	const auto truck_1 = truck{"bus", "red", 2008, 100'000, 30};
	const vehicle& vehicle_2 = truck_1;
	std::cout << vehicle_1 << "\n\n" << truck_1 << "\n\n" << vehicle_2;
}