numbers.cpp

#include <array>
#include <exception>
#include <iostream>

class BaseNumber {
public:
    virtual void print(std::ostream& os) const = 0;
};

template<typename T>
class Number : public BaseNumber {
protected:
    T n;
    Number(T n_) : n(n_) { }
public:
    T getValue() const { return n; }
    virtual void print(std::ostream& os) const { os << n; }
};

template<typename T>
std::ostream& operator <<(std::ostream& os, const Number<T>& n) { n.print(os); return os; }

class NaturalNumber : public Number<int>
{
public:
    NaturalNumber(int n_) : Number<int>(n_) { }
    virtual void print(std::ostream& os) const { std::cerr << "NaturalNumber::print()"; os << n; }
};

class Signed {
public:
    virtual void switchSign() = 0;
};

class IntegerNumber : public Number<int>, public Signed
{
public:
    IntegerNumber(int n_) : Number<int>(n_) {}
    virtual void print(std::ostream& os) const { std::cerr << "IntegerNumber::print()"; os << n; }
    virtual void switchSign() { n = -n; }
};

class RealNumber : public Number<double>, public Signed
{
public:
    RealNumber(double n_) : Number<double>(n_) {}
    virtual void print(std::ostream& os) const { std::cerr << "RealNumber::print()"; os << n; }
    virtual void switchSign() { n = -n; }
};


template<typename T>
Number<T> operator +(const Number<T>& r, const Number<T>& n) {
    return Number<T>(r.getValue() + n.getValue());
}

template<typename T>
RealNumber operator +(const RealNumber& r, const Number<T>& n) {
    return RealNumber(r.getValue() + n.getValue());
}
template<typename T>
RealNumber operator +(const Number<T>& n, const RealNumber& r) {
    return RealNumber(r.getValue() + n.getValue());
}

IntegerNumber operator +(const IntegerNumber& i, const NaturalNumber& n) {
    return IntegerNumber(i.getValue() + n.getValue());
}
IntegerNumber operator +(const NaturalNumber& n, const IntegerNumber& i) {
    return IntegerNumber(i.getValue() + n.getValue());
}

int main()
{
    NaturalNumber nn(12);
    RealNumber rn(4.2);
    rn.switchSign();
    const BaseNumber* pn1 = &nn;
    const BaseNumber* pn2 = &rn;
    std::cout << "Natural: " << nn << std::endl;
    std::cout << "Real: " << rn << std::endl;
    std::cout << "Sum: " << nn + rn << std::endl;
    return 0;
}