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<A NAME="Composite"></A>
<A NAME="intent"></A>
<H2><A HREF="#motivation"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Intent</H2>
<A NAME="auto1000"></A>
<P>Compose objects into tree structures to represent part-whole
hierarchies. Composite lets clients treat individual objects and
compositions of objects uniformly.</P>
<A NAME="motivation"></A>
<H2><A HREF="#applicability"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Motivation</H2>
<A NAME="auto1001"></A>
<P>Graphics applications like drawing editors and schematic capture
systems let users build complex diagrams out of simple components.
The user can group components to form larger components, which in
turn can be grouped to form still larger components. A simple
implementation could define classes for graphical primitives such
as Text and Lines plus other classes that act as containers for
these primitives.</P>
<A NAME="recursivecomp-graphics"></A>
<P>But there's a problem with this approach: Code that uses these
classes must treat primitive and container objects differently,
even if most of the time the user treats them identically. Having
to distinguish these objects makes the application more complex.
The Composite pattern describes how to use recursive composition
so that clients don't have to make this distinction.</P>
<A NAME="picture-163c"></A>
<P ALIGN=CENTER><IMG SRC="Pictures/compo075.gif"></P>
<A NAME="auto1002"></A>
<P>The key to the Composite pattern is an abstract class that
represents <EM>both</EM> primitives and their containers. For the
graphics system, this class is Graphic. Graphic declares operations
like Draw that are specific to graphical objects. It also declares
operations that all composite objects share, such as operations
for accessing and managing its children.</P>
<A NAME="auto1003"></A>
<P>The subclasses Line, Rectangle, and Text (see preceding class diagram)
define primitive graphical objects. These classes implement Draw to
draw lines, rectangles, and text, respectively. Since primitive
graphics have no child graphics, none of these subclasses implements
child-related operations.</P>
<A NAME="auto1004"></A>
<P>The Picture class defines an aggregate of Graphic objects. Picture
implements Draw to call Draw on its children, and it implements
child-related operations accordingly. Because the Picture interface
conforms to the Graphic interface, Picture objects can compose other
Pictures recursively.</P>
<A NAME="auto1005"></A>
<P>The following diagram shows a typical composite object structure
of recursively composed Graphic objects:</P>
<A NAME="picture-164o"></A>
<P ALIGN=CENTER><IMG SRC="Pictures/compo074.gif"></P>
<A NAME="applicability"></A>
<H2><A HREF="#structure"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Applicability</H2>
<A NAME="auto1006"></A>
<P>Use the Composite pattern when</P>
<UL>
<A NAME="auto1007"></A>
<LI>you want to represent part-whole hierarchies of objects.
<A NAME="auto1008"></A>
<P></P>
<A NAME="auto1009"></A>
<LI>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.
</UL>
<A NAME="structure"></A>
<H2><A HREF="#participants"><IMG SRC="gifsb/down3.gif" BORDER=0></A>
Structure</H2>
<P ALIGN=CENTER><IMG SRC="Pictures/compo072.gif"></P>
<A NAME="auto1010"></A>
<P>A typical Composite object structure might look like this:</P>
<A NAME="composite-inst"></A>
<P ALIGN=CENTER><IMG SRC="Pictures/compo073.gif"></P>
<A NAME="participants"></A>
<H2><A HREF="#collaborations"><IMG SRC="gifsb/down3.gif" BORDER=0></A>
Participants</H2>
<UL>
<A NAME="auto1011"></A>
<LI><B>Component</B> (Graphic)
<A NAME="auto1012"></A>
<P></P>
<UL>
<A NAME="auto1013"></A>
<LI>declares the interface for objects in the composition.</LI>
<A NAME="auto1014"></A>
<P><!-- extra space --></P>
<A NAME="auto1015"></A>
<LI>implements default behavior for the interface
common to all classes, as appropriate.</LI>
<A NAME="auto1016"></A>
<P><!-- extra space --></P>
<A NAME="auto1017"></A>
<LI>declares an interface for accessing and managing its child
components.</LI>
<A NAME="auto1018"></A>
<P><!-- extra space --></P>
<A NAME="auto1019"></A>
<LI>(optional) defines an interface for accessing a component's
parent in the recursive structure, and implements it if that's
appropriate.</LI>
</UL>
<A NAME="auto1020"></A>
<P></P>
<A NAME="leaf-part-comp"></A>
<LI><B>Leaf</B> (Rectangle, Line, Text, etc.)
<A NAME="auto1021"></A>
<P></P>
<UL>
<A NAME="auto1022"></A>
<LI>represents leaf objects in the composition. A leaf has no
children.</LI>
<A NAME="auto1023"></A>
<P><!-- extra space --></P>
<A NAME="auto1024"></A>
<LI>defines behavior for primitive objects in the composition.</LI>
</UL>
<A NAME="auto1025"></A>
<P></P>
<A NAME="auto1026"></A>
<LI><B>Composite</B> (Picture)
<A NAME="auto1027"></A>
<P></P>
<UL>
<A NAME="auto1028"></A>
<LI>defines behavior for components having children.</LI>
<A NAME="auto1029"></A>
<P><!-- extra space --></P>
<A NAME="auto1030"></A>
<LI>stores child components.</LI>
<A NAME="auto1031"></A>
<P><!-- extra space --></P>
<A NAME="auto1032"></A>
<LI>implements child-related operations in the Component interface.</LI>
</UL>
<A NAME="auto1033"></A>
<P></P>
<A NAME="auto1034"></A>
<LI><B>Client</B>
<A NAME="auto1035"></A>
<P></P>
<UL>
<A NAME="auto1036"></A>
<LI>manipulates objects in the composition through the
Component interface.</LI>
</UL>
</UL>
<A NAME="collaborations"></A>
<H2><A HREF="#consequences"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Collaborations</H2>
<UL>
<A NAME="auto1037"></A>
<LI>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.</LI>
</UL>
<A NAME="consequences"></A>
<H2><A HREF="#implementation"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Consequences</H2>
<A NAME="auto1038"></A>
<P>The Composite pattern</P>
<UL>
<A NAME="auto1039"></A>
<LI>defines class hierarchies consisting of primitive objects
and composite objects. Primitive objects can be composed into more
complex objects, which in turn can be composed, and so on recursively.
Wherever client code expects a primitive object, it can also take a
composite object.</LI>
<A NAME="auto1040"></A>
<P></P>
<A NAME="auto1041"></A>
<LI>makes the client simple.
Clients can treat composite structures and individual objects
uniformly. Clients normally don't know (and shouldn't care) whether
they're dealing with a leaf or a composite component. This simplifies
client code, because it avoids having to write
tag-and-case-statement-style functions over the classes that define
the composition.</LI>
<A NAME="auto1042"></A>
<P></P>
<A NAME="auto1043"></A>
<LI>makes it easier to add new kinds of components.
Newly defined Composite or Leaf subclasses work automatically with
existing structures and client code. Clients don't have to be changed
for new Component classes.</LI>
<A NAME="auto1044"></A>
<P></P>
<A NAME="auto1045"></A>
<LI>can make your design overly general.
The disadvantage of making it easy to add new components is that it
makes it harder to restrict the components of a composite. Sometimes
you want a composite to have only certain components. With
Composite, you can't rely on the type system to enforce those
constraints for you. You'll have to use run-time checks instead.</LI>
</UL>
<A NAME="implementation"></A>
<H2><A HREF="#samplecode"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Implementation</H2>
<A NAME="auto1046"></A>
<P>There are many issues to consider when implementing the Composite
pattern:</P>
<OL>
<A NAME="parentref-def-comp"></A>
<LI><EM>Explicit parent references.</EM>
Maintaining references from child components to their parent can
simplify the traversal and management of a composite structure. The
parent reference simplifies moving up the structure and deleting a
component. Parent references also help support the <A HREF="pat5afs.htm" TARGET="_mainDisplayFrame">Chain of Responsibility (223)</A> pattern.
<A NAME="auto1047"></A>
<P>The usual place to define the parent reference is in the Component
class. Leaf and Composite classes can inherit the reference and the
operations that manage it.</P>
<A NAME="auto1048"></A>
<P>With parent references, it's essential to maintain the invariant that
all children of a composite have as their parent the composite that in
turn has them as children. The easiest way to ensure this is to
change a component's parent <EM>only</EM> when it's being added or
removed from a composite. If this can be implemented once in the Add
and Remove operations of the Composite class, then it can be inherited
by all the subclasses, and the invariant will be maintained
automatically.</P>
</LI>
<A NAME="auto1049"></A>
<P></P>
<A NAME="auto1050"></A>
<LI><EM>Sharing components.</EM>
It's often useful to share components, for example, to reduce storage
requirements. But when a component can have no more than one parent,
sharing components becomes difficult.
<A NAME="flywt-w-compst"></A>
<P>A possible solution is for children to store multiple parents. But
that can lead to ambiguities as a request propagates up the structure.
The
<A HREF="pat4ffs.htm" TARGET="_mainDisplayFrame">Flyweight (195)</A> pattern shows how to rework a
design to avoid storing parents altogether. It works in cases where
children can avoid sending parent requests by externalizing some or
all of their state.</P>
</LI>
<A NAME="auto1051"></A>
<P></P>
<A NAME="auto1052"></A>
<LI><EM>Maximizing the Component interface.</EM>
One of the goals of the Composite pattern is to make clients unaware
of the specific Leaf or Composite classes they're using. To attain
this goal, the Component class should define as many common operations
for Composite and Leaf classes as possible. The Component class
usually provides default implementations for these operations, and
Leaf and Composite subclasses will override them.
<A NAME="auto1053"></A>
<P>However, this goal will sometimes conflict with the principle of class
hierarchy design that says a class should only define operations that
are meaningful to its subclasses. There are many operations that
Component supports that don't seem to make sense for Leaf classes.
How can Component provide a default implementation for them?</P>
<A NAME="auto1054"></A>
<P>Sometimes a little creativity shows how an operation that would appear
to make sense only for Composites can be implemented for all
Components by moving it to the Component class. For example, the
interface for accessing children is a fundamental part of a Composite
class but not necessarily Leaf classes. But if we view a Leaf as a
Component that <EM>never</EM> has children, then we can define a default
operation for child access in the Component class that never <EM>returns</EM> any children. Leaf classes can use the default
implementation, but Composite classes will reimplement it to return
their children.</P>
<A NAME="auto1055"></A>
<P>The child management operations are more troublesome and are discussed
in the next item.</P>
</LI>
<A NAME="auto1056"></A>
<P></P>
<A NAME="auto1057"></A>
<LI><EM>Declaring the child management operations.</EM>
Although the Composite class <EM>implements</EM> the Add and Remove
operations for managing children, an important issue in the Composite
pattern is which classes <EM>declare</EM> these operations in the
Composite class hierarchy. Should we declare these operations in the
Component and make them meaningful for Leaf classes, or should we
declare and define them only in Composite and its subclasses?
<A NAME="auto1058"></A>
<P>The decision involves a trade-off between safety and transparency:</P>
<UL>
<A NAME="auto1059"></A>
<LI>Defining the child management interface at the root of the class
hierarchy gives you transparency, because you can treat all components
uniformly. It costs you safety, however, because clients may try to
do meaningless things like add and remove objects from leaves.</LI>
<A NAME="auto1060"></A>
<P></P>
<A NAME="auto1061"></A>
<LI>Defining child management in the Composite class gives you safety,
because any attempt to add or remove objects from leaves will be
caught at compile-time in a statically typed language like C++. But
you lose transparency, because leaves and composites have different
interfaces.</LI>
</UL>
<A NAME="auto1062"></A>
<P>We have emphasized transparency over safety in this pattern. If you
opt for safety, then at times you may lose type information and have
to convert a component into a composite. How can you do this without
resorting to a type-unsafe cast?</P>
<A NAME="leaf-in-comp"></A>
<P>One approach is to declare an operation <CODE>Composite*
GetComposite()</CODE> in the Component class. Component provides a default
operation that returns a null pointer. The Composite class redefines
this operation to return itself through the <CODE>this</CODE> pointer:</P>
<A NAME="auto1063"></A>
<PRE>
class Composite;
class Component {
public:
//...
virtual Composite* GetComposite() { return 0; }
};
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