// This code demonstrates polymorphism.  foo() doesn’t know what
// particular sort of Readable it’s dealing with, but it calls
// methods whose implementations vary amongst the derived classes.
//
// The oi() method is just there to make it harder.

#include <iostream>
#include <iomanip>

using namespace std;

// Something you can read.  Could be a comic book, a billboard,
// a computer display, etc.

class Readable {
  public:
    virtual void oi() const { cout << "Hello there!\n"; }
    virtual int power_consumption() const = 0;	// mW/hour
    virtual float cost() const = 0;		// U.S. Dollars
};

void foo(const Readable &thing) {
    thing.oi();
    cout << fixed << setprecision(2)
         << "This thing costs $" << thing.cost()
         << " and consumes " << thing.power_consumption() << " mW/h\n";
}

// This code might be in a whole different file:

class Book : public Readable {
  public:
    virtual int power_consumption() const override { return 0; }
    virtual float cost() const override { return 3.00; }
};

// Remember that override is optional, and virtual is inherited.
class Ipad : public Readable {
  public:
    int power_consumption() const { return 17; }	// I made this up.
    float cost() const { return 219.99; }		// Amazon.com, refurb
};

int main() {
    Book b;
    Ipad i;
    foo(b);
    foo(i);
}