abstract.cpp

#include <iostream>
using namespace std;

class Device {
    public:
       void turn_on() {
            cout << "Device is on." << endl;
        }
        virtual void say_hello() = 0;
};

class Laptop: public Device {
    public:
       void say_hello() {
            cout << "Hello world!" << endl;
        }
};

int main() {
    Laptop Laptop_instance;
    Laptop_instance.turn_on();
    Laptop_instance.say_hello();

    // Device Device_instance;
    // will cause compile time error
    return 0;
}