Flyweight - shoonie/StudyingDesignPattern GitHub Wiki

Intent

Use sharing to support large numbers of fine-grained objects efficiently.

Diagram

disgram

Consequences

Flyweights may introduce run-time costs associated with transferring, finding, and/or computing extrinsic state especially if it was formerly stored as intrinsic state. However, such costs are offset by space savings, which increase as more flyweights are shared. Storage savings are a function of several factors.

the reduction in the total number of instances that comes from sharing
the amount of intrinsic state per object.
whether extrinsic state is computed or state. The more flyweights are shared, the greater the storage savings.

Implementation

Removing extrinsic state.
Managing shared objects.

Sample Code

// from https://advancedcppwithexamples.blogspot.com/2010/10/c-example-of-flyweight-design-pattern.html
#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;
}
// also many examples of making lots of trees in the game.
// check http://gameprogrammingpatterns.com/flyweight.html