Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Multiple dispatching

When dealing with multiple types which are interacting, a program can get particularly messy. For example, consider a system that parses and executes mathematical expressions. You want to be able to say Number + Number , Number * Number , etc., where Number is the base class for a family of numerical objects. But when you say a + b , and you don’t know the exact type of either a or b, so how can you get them to interact properly?

The answer starts with something you probably don’t think about: C++ performs only single dispatching. That is, if you are performing an operation on more than one object whose type is unknown, C++ can invoke the dynamic binding mechanism on only one of those types. This doesn’t solve the problem, so you end up detecting some types manually and effectively producing your own dynamic binding behavior.

The solution is called multiple dispatching . Remember that polymorphism can occur only via member function calls, so if you want double dispatching to occur, there must be two member function calls: the first to determine the first unknown type, and the second to determine the second unknown type. With multiple dispatching, you must have a virtual call to determine each of the types. Generally, you’ll set up a configuration such that a single member function call produces more than one dynamic member function call and thus determines more than one type in the process. To get this effect, you need to work with more than one virtual function: you’ll need a virtual function call for each dispatch. The virtual functions in the following example are called compete( ) and eval( ), and are both members of the same type. (In this case there will be only two dispatches, which is referred to as double dispatching ). If you are working with two different type hierarchies that are interacting, then you’ll have to have a virtual call in each hierarchy.

Here’s an example of multiple dispatching: 

//: C25:PaperScissorsRock.cpp
// Demonstration of multiple dispatching
#include "../purge.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;

class Paper;
class Scissors;
class Rock;

enum Outcome { win, lose, draw };

ostream& 
operator<<(ostream& os, const Outcome out) {
  switch(out) {
    default:
    case win: return os << "win";
    case lose: return os << "lose";
    case draw: return os << "draw";
  }
}

class Item {
public:
  virtual Outcome compete(const Item*) = 0;
  virtual Outcome eval(const Paper*) const = 0;
  virtual Outcome eval(const Scissors*) const= 0;
  virtual Outcome eval(const Rock*) const = 0;
  virtual ostream& print(ostream& os) const = 0;
  virtual ~Item() {}
  friend ostream& 
  operator<<(ostream& os, const Item* it) {
    return it->print(os);
  }
};

class Paper : public Item {
public:
  Outcome compete(const Item* it) {
    return it->eval(this);
  }
  Outcome eval(const Paper*) const {
    return draw;
  }
  Outcome eval(const Scissors*) const {
    return win;
  }
  Outcome eval(const Rock*) const {
    return lose;
  }
  ostream& print(ostream& os) const {
    return os << "Paper   ";
  }
};

class Scissors : public Item {
public:
  Outcome compete(const Item* it) {
    return it->eval(this);
  }
  Outcome eval(const Paper*) const {
    return lose;
  }
  Outcome eval(const Scissors*) const {
    return draw;
  }
  Outcome eval(const Rock*) const {
    return win;
  }
  ostream& print(ostream& os) const {
    return os << "Scissors";
  }
};

class Rock : public Item {
public:
  Outcome compete(const Item* it) {
    return it->eval(this);
  }
  Outcome eval(const Paper*) const {
    return win;
  }
  Outcome eval(const Scissors*) const {
    return lose;
  }
  Outcome eval(const Rock*) const {
    return draw;
  }
  ostream& print(ostream& os) const {
    return os << "Rock    ";
  }
};

struct ItemGen {
  ItemGen() { srand(time(0)); }
  Item* operator()() {
    switch(rand() % 3) {
      default:
      case 0:
        return new Scissors();
      case 1:
        return new Paper();
      case 2:
        return new Rock();
    }
  }
};

struct Compete {
  Outcome operator()(Item* a, Item* b) {
    cout << a << "\t" << b << "\t";
    return a->compete(b);
  }
};

int main() {
  const int sz = 20;
  vector<Item*> v(sz*2);
  generate(v.begin(), v.end(), ItemGen());
  transform(v.begin(), v.begin() + sz, 
    v.begin() + sz, 
    ostream_iterator<Outcome>(cout, "\n"), 
    Compete());
  purge(v);
} ///:~

Visitor, a type of multiple dispatching

The assumption is that you have a primary class hierarchy that is fixed; perhaps it’s from another vendor and you can’t make changes to that hierarchy. However, you’d like to add new polymorphic methods to that hierarchy, which means that normally you’d have to add something to the base class interface. So the dilemma is that you need to add methods to the base class, but you can’t touch the base class. How do you get around this?

The design pattern that solves this kind of problem is called a “visitor” (the final one in the Design Patterns book), and it builds on the double dispatching scheme shown in the last section.

The visitor pattern allows you to extend the interface of the primary type by creating a separate class hierarchy of type Visitor to virtualize the operations performed upon the primary type. The objects of the primary type simply “accept” the visitor, then call the visitor’s dynamically -bound member function. 

//: C25:BeeAndFlowers.cpp
// Demonstration of "visitor" pattern
#include "../purge.h"
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;

class Gladiolus;
class Renuculus;
class Chrysanthemum;

class Visitor {
public:
  virtual void visit(Gladiolus* f) = 0;
  virtual void visit(Renuculus* f) = 0;
  virtual void visit(Chrysanthemum* f) = 0;
  virtual ~Visitor() {}
};

class Flower {
public:
  virtual void accept(Visitor&) = 0;
  virtual ~Flower() {}
};

class Gladiolus : public Flower {
public:
  virtual void accept(Visitor& v) {
    v.visit(this);
  }
};

class Renuculus : public Flower {
public:
  virtual void accept(Visitor& v) {
    v.visit(this);
  }
};

class Chrysanthemum : public Flower {
public:
  virtual void accept(Visitor& v) {
    v.visit(this);
  }
};

// Add the ability to produce a string:
class StringVal : public Visitor {
  string s;
public:
  operator const string&() { return s; }
  virtual void visit(Gladiolus*) {
    s = "Gladiolus";
  }
  virtual void visit(Renuculus*) {
    s = "Renuculus";
  }
  virtual void visit(Chrysanthemum*) {
    s = "Chrysanthemum";
  }
};

// Add the ability to do "Bee" activities:
class Bee : public Visitor {
public:
  virtual void visit(Gladiolus*) {
    cout << "Bee and Gladiolus\n";
  }
  virtual void visit(Renuculus*) {
    cout << "Bee and Renuculus\n";
  }
  virtual void visit(Chrysanthemum*) {
    cout << "Bee and Chrysanthemum\n";
  }
};

struct FlowerGen {
  FlowerGen() { srand(time(0)); }
  Flower* operator()() {
    switch(rand() % 3) {
      default:
      case 0: return new Gladiolus();
      case 1: return new Renuculus();
      case 2: return new Chrysanthemum();
    }
  }
};

int main() {
  vector<Flower*> v(10);
  generate(v.begin(), v.end(), FlowerGen());
  vector<Flower*>::iterator it;
  // It's almost as if I added a virtual function
  // to produce a Flower string representation:
  StringVal sval;
  for(it = v.begin(); it != v.end(); it++) {
    (*it)->accept(sval);
    cout << string(sval) << endl;
  }
  // Perform "Bee" operation on all Flowers:
  Bee bee;
  for(it = v.begin(); it != v.end(); it++)
    (*it)->accept(bee);
  purge(v);
} ///:~

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