Double dispatch in C++

double-dispatch.cpp

#include <stdio.h>

/* Forward declarations. */
class Structure;
class Plane;
class Cylinder;
void print2(Cylinder*, Plane*);
void print2(Plane*, Cylinder*);
void print2(Cylinder*, Cylinder*);
void print2(Plane*, Plane*);

class Structure {
        public:
                virtual void print(Structure* target) {};
                virtual void print_helper(Plane* origin) {};
                virtual void print_helper(Cylinder* origin) {};
};

class Plane : public Structure {
        public:
                Plane() : name("Plane") {};
                virtual void print(Structure* target) {
                        printf("Plane::print --- ");
                        target->print_helper(this);
                }
                virtual void print_helper(Plane* origin) {
                        ::print2(origin, this);
                }
                virtual void print_helper(Cylinder* origin) {
                        ::print2(origin, this);
                }
                const char* name;
};

class Cylinder : public Structure {
        public:
                Cylinder() : name("Cylinder") {};
                virtual void print(Structure* target) {
                        printf("Cylinder::print --- ");
                        target->print_helper(this);
                }
                virtual void print_helper(Plane* origin) {
                        ::print2(origin, this);
                }
                virtual void print_helper(Cylinder* origin) {
                        ::print2(origin, this);
                }
                const char* name;
};

void print2(Cylinder* c, Plane* p) {
        printf("%s %s\n", c->name, p->name);
}
void print2(Plane* p, Cylinder* c) {
        printf("%s %s\n", p->name, c->name);
}
void print2(Cylinder* c1, Cylinder* c2) {
        printf("%s %s\n", c1->name, c2->name);
}
void print2(Plane* p1, Plane* p2) {
        printf("%s %s\n", p1->name, p2->name);
}

int main(void) {
        Plane p;
        Cylinder c;

        Cylinder* c_ptr = &c;
        Plane* p_ptr = &p;

        printf("*** Single dispatch ***\n");
        c.print_helper(p_ptr);
        p.print_helper(c_ptr);
        c.print_helper(c_ptr);
        p.print_helper(p_ptr);

        printf("*** Double dispatch ***\n");
        c.print((Structure *) p_ptr);
        p.print((Structure *) c_ptr);
        c.print((Structure *) c_ptr);
        p.print((Structure *) p_ptr);

        return 0;
}