1. Usage

you need to use many objects lets put 1000-100000. They are similar and related. For example icons of folders and files in Windows. To allow them share similar properties from one side but be ready to be customized we can use Flyweight pattern. Inner proprieties shared by all objects of group called Intrinsic statewhile operational properties made at customization phase called Extrinsic state

2. UML class diagram

(Source: http://www.cs.unc.edu/~stotts/GOF/hires/pat4ffso.htm)

It is important to understand:

• Flyweight- declares an interface through which flyweights can receive and act on extrinsic state.
• ConcreteFlyweight (Icon)- implements the Flyweight interface and adds storage for intrinsic state, if any. A ConcreteFlyweight object must be sharable. Any state it stores must be intrinsic; that is, it must be independent of the ConcreteFlyweight object’s context.
• FlyweightFactory – creates and manages flyweight objects. ensures that flyweights are shared properly. When a client requests a flyweight, the FlyweightFactory object supplies an existing instance or creates one, if none exists.
• Client – maintains a reference to flyweight(s). computes or stores the extrinsic state of flyweight(s).

3. Pros

Allow avoid the overhead of large numbers of very similar classes.

4. Cons

Can introduce real-time cost associated with transferring, finding and/or computing extrinsic state, especially if it was formerly stored as intrinsic state.

5. Source code

// From https://sourcemaking.com/design_patterns/flyweight/cpp/2
#include <iostream.h>
#include <string.h>

class Icon
{
  public:
    Icon(char *fileName)
    {
        strcpy(_name, fileName);
        if (!strcmp(fileName, "go"))
        {
            _width = 20;
            _height = 20;
        }
        if (!strcmp(fileName, "stop"))
        {
            _width = 40;
            _height = 40;
        }
        if (!strcmp(fileName, "select"))
        {
            _width = 60;
            _height = 60;
        }
        if (!strcmp(fileName, "undo"))
        {
            _width = 30;
            _height = 30;
        }
    }
    const char *getName()
    {
        return _name;
    }
    draw(int x, int y)
    {
        cout << "   drawing " << _name << ": upper left (" << x << "," << y << 
          ") - lower right (" << x + _width << "," << y + _height << ")" <<
          endl;
    }
  private:
    char _name[20];
    int _width;
    int _height;
};

class FlyweightFactory
{
  public:
    static Icon *getIcon(char *name)
    {
        for (int i = 0; i < _numIcons; i++)
          if (!strcmp(name, _icons[i]->getName()))
            return _icons[i];
        _icons[_numIcons] = new Icon(name);
        return _icons[_numIcons++];
    }
    static void reportTheIcons()
    {
        cout << "Active Flyweights: ";
        for (int i = 0; i < _numIcons; i++)
          cout << _icons[i]->getName() << " ";
        cout << endl;
    }
  private:
    enum
    {
        MAX_ICONS = 5
    };
    static int _numIcons;
    static Icon *_icons[MAX_ICONS];
};

int FlyweightFactory::_numIcons = 0;
Icon *FlyweightFactory::_icons[];

class DialogBox
{
  public:
    DialogBox(int x, int y, int incr): _iconsOriginX(x), _iconsOriginY(y),
      _iconsXIncrement(incr){}
    virtual void draw() = 0;
  protected:
    Icon *_icons[3];
    int _iconsOriginX;
    int _iconsOriginY;
    int _iconsXIncrement;
};

class FileSelection: public DialogBox
{
  public:
    FileSelection(Icon *first, Icon *second, Icon *third): DialogBox(100, 100,
      100)
    {
        _icons[0] = first;
        _icons[1] = second;
        _icons[2] = third;
    }
    void draw()
    {
        cout << "drawing FileSelection:" << endl;
        for (int i = 0; i < 3; i++)
          _icons[i]->draw(_iconsOriginX + (i *_iconsXIncrement), _iconsOriginY);
    }
};

class CommitTransaction: public DialogBox
{
  public:
    CommitTransaction(Icon *first, Icon *second, Icon *third): 
        DialogBox(150, 150, 150)
    {
        _icons[0] = first;
        _icons[1] = second;
        _icons[2] = third;
    }
    void draw()
    {
        cout << "drawing CommitTransaction:" << endl;
        for (int i = 0; i < 3; i++)
          _icons[i]->draw(_iconsOriginX + (i *_iconsXIncrement), _iconsOriginY);
    }
};

int main()
{
  DialogBox *dialogs[2];
  dialogs[0] = new FileSelection(FlyweightFactory::getIcon("go"),
    FlyweightFactory::getIcon("stop"), FlyweightFactory::getIcon("select"));
  dialogs[1] = new CommitTransaction(FlyweightFactory::getIcon("select"),
    FlyweightFactory::getIcon("stop"), FlyweightFactory::getIcon("undo"));

  for (int i = 0; i < 2; i++)
    dialogs[i]->draw();

  FlyweightFactory::reportTheIcons();
}

6. Additional Info

• Boost library has Module Boost.Flyweight