Advanced C++ with Examples: C++ example of Flyweight Design Pattern

Wednesday 6 October 2010

C++ example of Flyweight Design Pattern

The Flyweight pattern describes how to share objects to allow their use at fine granularities without prohibitive cost. Each “flyweight” object is divided into two pieces: the state-dependent (extrinsic) part, and the state-independent (intrinsic) part. Intrinsic state is stored (shared) in the Flyweight object. Extrinsic state is stored or computed by client objects, and passed to the Flyweight when its operations are invoked.

The Ant, Locust, and Cockroach classes can be “light-weight” because their instance-specific state has been de-encapsulated, or externalized, and must be supplied by the client.

The frequency of use of the flyweight pattern is low.

The following example illustrates the Flyweight pattern:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Flyweight is part of Structural Patterns
//Structural Patterns deal with decoupling the interface and implementation of classes and objects
//A Flyweight uses sharing to support large numbers of fine-grained objects efficiently. 

//We will take an example of charachter class. Each charachter is unique and can have different size
//but the rest of the features will remain the same.

#include<iostream>
#include<string>
#include<map>

using namespace std;

// The 'Flyweight' abstract class
class Character
{
public:
  virtual void Display(int pointSize) = 0;

protected:
  char symbol_;
  int width_;
  int height_;
  int ascent_;
  int descent_;
  int pointSize_;
};

// A 'ConcreteFlyweight' class
class CharacterA : public Character
{
public:
  CharacterA()
  {
    symbol_    = 'A';
    width_     = 120;
    height_    = 100;
    ascent_    = 70;
    descent_   = 0;
    pointSize_ = 0; //initialise
  }
  void Display(int pointSize)
  {
    pointSize_ = pointSize;
    cout<<symbol_<<" (pointsize "<<pointSize_<<" )"<<endl;
  }
};

// A 'ConcreteFlyweight' class
class CharacterB : public Character
{
public:
  CharacterB()
  {
    symbol_    = 'B';
    width_     = 140;
    height_    = 100;
    ascent_    = 72;
    descent_   = 0;
    pointSize_ = 0; //initialise
  }
  void Display(int pointSize)
  {
    pointSize_ = pointSize;
    cout<<symbol_<<" (pointsize "<<pointSize_<<" )"<<endl;
  }
};

//C, D, E,...

// A 'ConcreteFlyweight' class
class CharacterZ : public Character
{
public:
  CharacterZ()
  {
    symbol_    = 'Z';
    width_     = 100;
    height_    = 100;
    ascent_    = 68;
    descent_   = 0;
    pointSize_ = 0; //initialise
  }
  void Display(int pointSize)
  {
    pointSize_ = pointSize;
    cout<<symbol_<<" (pointsize "<<pointSize_<<" )"<<endl;
  }
};

// The 'FlyweightFactory' class
class CharacterFactory
{
public:
  virtual ~CharacterFactory()
  {
    while(!characters_.empty())
    {
      map<char, Character*>::iterator it = characters_.begin();
      delete it->second;
      characters_.erase(it);
    }
  }
  Character* GetCharacter(char key)
  {
    Character* character = NULL;
    if(characters_.find(key) != characters_.end())
    {
      character = characters_[key];
    }
    else
    {
      switch(key)
      {
      case 'A':
        character = new CharacterA();
        break;
      case 'B':
        character = new CharacterB();
        break;
      //...
      case 'Z':
        character = new CharacterZ();
        break;
      default:
        cout<<"Not Implemented"<<endl;
        throw("Not Implemented");
      }
      characters_[key] = character;
    }
    return character;
  }
private:
  map<char, Character*> characters_;
};


//The Main method
int main()
{
  string document = "AAZZBBZB";
  const char* chars = document.c_str();

  CharacterFactory* factory = new CharacterFactory;

  // extrinsic state
  int pointSize = 10;

  // For each character use a flyweight object
  for(size_t i = 0; i < document.length(); i++)
  {
    pointSize++;
    Character* character = factory->GetCharacter(chars[i]);
    character->Display(pointSize);
  }

  //Clean memory
  delete factory;

  return 0;
}