Visitor Pattern demo in C++ using std::visit and std::variant

visitorPatternDemo.cpp

#include <iostream>
#include <variant>
#include <cstdlib>

using Real = double;

struct Rational {
    int numerator;
    int denominator;
    
    Rational(int numerator, int denominator)
        : numerator{numerator}, denominator{denominator} {};
};

using Number = std::variant<Real, Rational>;

struct PrintVisitor
{
    void operator()(const Real value) const
        { std::cout << "value: " << value << std::endl; }    
        
    void operator()(const Rational rt) const
        { std::cout << "value: " << rt.numerator
                    << "/" << rt.denominator << std::endl; }
};

struct SumVisitor
{
    Number operator()(Real r1, Real r2) const
        { return r1 + r2; }
        
    Number operator()(Rational rt1, Rational rt2) const
        { return Rational{
                rt1.numerator*rt2.denominator + rt2.numerator*rt1.denominator,
                rt1.denominator * rt2.denominator }; }        
        
    Number operator()(Rational rt, Real r) const
        { return Real{(r*rt.denominator + rt.numerator)/rt.denominator}; }        
        
    Number operator()(Real r, Rational rt) const
        { return operator()(rt, r); }
};


int main()
{    
    const auto _minus_two_thirds = Number{std::in_place_type<Rational>, -2., 3.};
    const auto _22_over_15 = Number{std::in_place_type<Rational>, 22., 15.};
    const auto _29_point_9 = Number{std::in_place_type<Real>, 29.9};
    const auto _11_point_3 = Number{std::in_place_type<Real>, 11.3};    
    
    const auto alue = std::visit(SumVisitor{},
                            _11_point_3,
                            std::visit(SumVisitor{},
                                        _29_point_9,
                                        std::visit(SumVisitor{},
                                                    _22_over_15,
                                                    _minus_two_thirds)));
    
    std::visit(PrintVisitor{}, alue);
};