Layering & Contract Philosophy With additional indirection

 class CComponent
{
public: virtual void Operation() = ;
public: virtual void AddComponent(Component* p)= ;
public: virtual void RemoveComponent(Component* p)= ;
public: virtual Component* GetChild(int i) { return NULL;}
}
class CComposite: public CComponent
{
public: virtual void Operation()
{ foreach ( i ) GetChild(i)->Opeartion(); };
public: virtual void AddComponent(Component* p)
{ v.add( p );};
public: virtual void RemoveComponent(Component* p)
{ v.remove(p);}
public: virtual Component* GetChild(int i)
{ return v[i];}
private: vector< CComponent* > v;
}
class CLeaf: public CCompoent
{
public: virtual void Operation()
{ do_something_for_leaf(); };
}
class Client
{
CLeaf* pleaf1 = new CLeaf; CLeaf* pleaf2 = new CLeaf;
CLeaf* pleaf3 = new CLeaf; CLeaf* pleaf4 = new CLeaf;
CLeaf* pleaf5 = new CLeaf; CLeaf* pleaf6 = new CLeaf;
CComposite* pcomposite1, pcomposite2 = new CComposite;
pcomposite1.addcomponent(pleaf1);
pcomposite1.addcomponent(pleaf2);
pcomposite1.addcomponent(pleaf3);
pcomposite2.addcomponent(pleaf4);
pcomposite2.addcomponent(pleaf5);
pcomposite2.addcomponent(pcomposite1);
CComposite* pAll = new CComposite;
pAll.addComponent(pcomposite1);
pAll.addComponent(pcomposite2);
pAll.addComponent(pleaf6);
pAll.Opearation();
}

Applicability

Use the Composite pattern when:

  • you want to represent part-whole hierarchies of objects.
  • you want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly.

Participants

Component (Graphic)

  • declares the interface for objects in the composition.
  • implements default behavior for the interface common to all classes, as appropriate.
  • declares an interface for accessing and managing its child components.
  • (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.

Leaf (Rectangle, Line, Text, etc.)

  • represents leaf objects in the composition. A leaf has no children.
  • defines behavior for primitive objects in the composition.

Composite (Picture)

  • defines behavior for components having children.
  • stores child components.
  • implements child-related operations in the Component interface.

Client

  • manipulates objects in the composition through the Component interface.

Collaborations

  • Clients use the Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding.

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