visitor.cpp

// see https://www.modernescpp.com/index.php/the-visitor-pattern

#include <iostream>
#include <string>
#include <vector>

class CarElementVisitor;

class CarElement {                                     // (5)       
 public:
    virtual void accept(CarElementVisitor& visitor) const = 0;
    virtual ~CarElement() = default;
};

class Body;
class Car;
class Engine;
class Wheel;

class CarElementVisitor {                              // (6) 
 public:
    virtual void visit(Body body) const = 0;
    virtual void visit(Car car) const = 0;
    virtual void visit(Engine engine) const = 0;
    virtual void visit(Wheel wheel) const = 0;
    virtual ~CarElementVisitor() = default;
};

class Wheel: public CarElement {
 public:
    Wheel(const std::string& n): name(n) { }

    void accept(CarElementVisitor& visitor) const override {
        visitor.visit(*this);
    }

    std::string getName() const {
        return name;
    }
 private:
    std::string name;
};

class Body: public CarElement {
 public:
    void accept(CarElementVisitor& visitor) const override {
        visitor.visit(*this);
    }
};

class Engine: public CarElement {
 public:
    void accept(CarElementVisitor& visitor) const override {
        visitor.visit(*this);
    }
};

class Car: public CarElement {
 public:
    Car(std::initializer_list<CarElement*> carElements ): elements{carElements} {}
   
    void accept(CarElementVisitor& visitor) const override {
        for (auto elem : elements) {
            elem->accept(visitor);
        }
        visitor.visit(*this);
    }
 private:
    std::vector<CarElement*> elements;                   // (7) 
};

class CarElementDoVisitor: public CarElementVisitor {
   
    void visit(Body body) const override {
        std::cout << "Moving my body" << '\n';
    }

     void visit(Car car) const override {
        std::cout << "Starting my car" << '\n';
    }

    void visit(Wheel wheel) const override {
        std::cout << "Kicking my " << wheel.getName() << " wheel" << '\n';
    }

    void visit(Engine engine) const override {
        std::cout << "Starting my engine" << '\n';
    }
};

class CarElementPrintVisitor: public CarElementVisitor {
   
    void visit(Body body) const override {
        std::cout << "Visiting body" << '\n';
    }

     void visit(Car car) const override {
        std::cout << "Visiting car" << '\n';
    }

    void visit(Wheel wheel) const override {
        std::cout << "Visiting " << wheel.getName() << " wheel" << '\n';
    }

    void visit(Engine engine) const override {
        std::cout << "Visiting engine" << '\n';
    }
};

int main() {

    std::cout << '\n';

    Wheel wheelFrontLeft("front left");        
    Wheel wheelFrontRight("front right");
    Wheel wheelBackLeft("back left");
    Wheel wheelBackRight("back right");
    Body body;
    Engine engine;
    Car car {&wheelFrontLeft, &wheelFrontRight, &wheelBackLeft, &wheelBackRight,
             &body, &engine};

    CarElementPrintVisitor carElementPrintVisitor; 

    engine.accept(carElementPrintVisitor);       // (1)              
    car.accept(carElementPrintVisitor);          // (2)          

    std::cout << '\n';

    CarElementDoVisitor carElementDoVisitor;

    engine.accept(carElementDoVisitor);          // (3)             
    car.accept(carElementDoVisitor);             // (4)        

    std::cout << '\n';

}