Pure Virtual Function

#include <iostream>
#include <cstdlib>
#include <typeinfo>

using namespace std;

class Unit{
    protected:
        double value;
    public:
        virtual double input() = 0;
        virtual double convert() = 0;
        virtual void display() = 0;
        virtual double getValue() = 0;
};

class Farenheit: public Unit{
    public:
        double input();
        // Converts from Farenheit to Celsius
        double convert();
        void display();
        double getValue();
};

class Celsius: public Unit{
    public:
        double input();
        // Converts from Celsius to Farenheit
        double convert();
        void display();
        double getValue();
};

class Litres: public Unit{
    public:
        double input();
        // Converts from Litres to Galons
        double convert();
        void display();
        double getValue();
};

class Galons: public Unit{
    public:
        double input();
        // Converts from Galons to Litres
        double convert();
        void display();
        double getValue();
};

double Farenheit::convert(){
    return (5.0 / 9.0) * (value - 32);
}

double Celsius::convert(){
    return ((9.0 / 5.0) * value) + 32;
}

double Litres::convert(){
    return value / 3.875;
}

double Galons::convert(){
    return value * 3.875;
}

double Farenheit::input(){
    cout << "Enter value in F = ";
    cin >> value;
}

double Celsius::input(){
    cout << "Enter value in C = ";
    cin >> value;
}

double Litres::input(){
    cout << "Enter value in L = ";
    cin >> value;
}

double Galons::input(){
    cout << "Enter value in G = ";
    cin >> value;
}

void Farenheit::display(){
    cout << value << "F" << endl;
}

void Celsius::display(){
    cout << value << "C" << endl;
}

void Litres::display(){
    cout << value << "L" << endl;
}

void Galons::display(){
    cout << value << "G" << endl;
}

double Farenheit::getValue(){
    return value;
}

double Celsius::getValue(){
    return value;
}

double Litres::getValue(){
    return value;
}

double Galons::getValue(){
    return value;
}

void add(Unit *val1, Unit *val2){
    if(typeid(*val1) == typeid(*val2)){
        cout << " ";
        val1->display();
        cout << "+";
        val2->display();
        cout << "=";
        cout << val1->getValue() + val2->getValue() << endl;
    }
    else{
        cout << " ";
        val1->display();
        cout << "+";
        val2->display();
        cout << "=";
        cout << val1->convert() + val2->getValue() << endl;
    }
}

int main() {
    system("cls");
    Celsius cel;
    Farenheit far;

    cel.input();
    far.input();

    add(&cel, &far);
    cout << endl;
    add(&far, &cel);
    cout << endl;
    add(&far, &far);
    cout << endl;
    add(&cel, &cel);

    cout << endl;
    system("pause");
    return 0;
}