Composite

Used to implement a hierarchy of components, with leaves, a root, and subtrees. Client calls to a component function recursively call the same function for its sub-components, or just the function for itself if the call is on a leaf node.

Use when:

• Building parts-of-a-whole with components with a hierarchy
• Clients should ignore/not care where they are within the hierarchy to act upon a component and all of its subcomponents (if there are any)

Example C++ Code

// component interface (root type)
class Equipment
{
public:
    virtual ~Equipment();

    const char* Name() { return m_name; }

    // the 'operation' that all components have
    virtual Currency Price();

    // provides tree structure util funcs
    virtual void Add(Equipment*);
    virtual void Remove(Equipment*);
    virtual Iterator<Equipment*>* CreateIterator(); // for traversing all children
protected:
    Equipment(const char*);
private:
    const char* m_name;
};

// a leaf node type that has no sub-components
class FloppyDisk : public Equipment
{
public:
    FloppyDisk(const char*);
    virtual ~FloppyDisk();

    // 'Operation()' implementation
    virtual Currency Price();
};

// the Composite which contains sub-components
class CompositeEquipment : public Equipment
{
public:

    // will iterate and add price of all sub-children
    virtual Currency Price()
    {
        Iterator<Equipment*>* i = CreateIterator();
        Currency total = 0;

        for (i->First(); !i->IsDone(); i++)
        {
            total += i->CurrentItem()->Price();
        }
        
        delete i;
        return total;
    }

    // provides tree structure util funcs
    virtual void Add(Equipment*);
    virtual void Remove(Equipment*);
    virtual Iterator<Equipment*>* CreateIterator(); // for traversing all children
protected:
    CompositeEquipment(const char*);
private:
    List<Equipment*>* m_equipment;
};

// implementation of a composite, parent node
class ComputerChassis : public CompositeEquipment
{
public:
    ComputerChassis(const char*);
    virtual ~ComputerChassis();

    virtual Currency Price();
};