/**
 * "Subject"
 */
class IMath {
public:
    virtual double add(double x, double y) = 0;
    virtual double sub(double x, double y) = 0;
    virtual double mul(double x, double y) = 0;
    virtual double div(double x, double y) = 0;
};

/**
 * "Real Subject"
 */
class Math : public IMath {
public:
    double add(double x, double y) {
        return x + y;
    }

    double sub(double x, double y) {
        return x - y;
    }

    double mul(double x, double y) {
        return x * y;
    }

    double div(double x, double y) {
        return x / y;
    }
};

/**
 * "Proxy Object"
 */
class MathProxy : public IMath {
public:
    double add(double x, double y) {
        return math.add(x, y);
    }

    double sub(double x, double y) {
        return math.sub(x, y);
    }

    double mul(double x, double y) {
        return math.mul(x, y);
    }

    double div(double x, double y) {
        return math.div(x, y);
    }

private:
    Math math;
};

#include <iostream>

using std::cout;
using std::endl;

int main() {

    // Create math proxy
    MathProxy p;

    // Do the math
    cout << "4 + 2 = " << p.add(4, 2) << endl;
    cout << "4 - 2 = " << p.sub(4, 2) << endl;
    cout << "4 * 2 = " << p.mul(4, 2) << endl;
    cout << "4 / 2 = " << p.div(4, 2) << endl;

    return 0;
}