BOOST_OPENMETHOD(
    encounter,
    (std::ostream&, virtual_ptr<Animal>, virtual_ptr<Animal>), void);

// 'encounter' catch-all implementation.
BOOST_OPENMETHOD_OVERRIDE(
    encounter,
    (std::ostream & os, virtual_ptr<Animal> a, virtual_ptr<Animal> b), void) {
    os << a->name << " and " << b->name << " ignore each other";
}

// Add definitions for specific pairs of animals.
BOOST_OPENMETHOD_OVERRIDE(
    encounter,
    (std::ostream & os, virtual_ptr<Dog> /*dog1*/, virtual_ptr<Dog> /*dog2*/), void) {
    os << "Both wag tails";
}

BOOST_OPENMETHOD_OVERRIDE(
    encounter, (std::ostream & os, virtual_ptr<Dog> dog, virtual_ptr<Cat> cat),
    void) {
    os << dog->name << " chases " << cat->name;
}

BOOST_OPENMETHOD_OVERRIDE(
    encounter, (std::ostream & os, virtual_ptr<Cat> cat, virtual_ptr<Dog> dog),
    void) {
    os << cat->name << " runs away from " << dog->name;
}

// cat and dog
encounter(std::cout, *felix, *snoopy); // Felix runs away from Snoopy
std::cout << ".\n";

// dog and cat
encounter(std::cout, *snoopy, *felix); // Snoopy chases Felix
std::cout << ".\n";

// dog and dog
encounter(std::cout, *snoopy, *hector); // Both wag tails
std::cout << ".\n";

// cat and cat
std::unique_ptr<Animal> tom(new Cat("Tom"));
encounter(std::cout, *felix, *tom); // Felix and Tom ignore each other
std::cout << ".\n";