Composite pattern

Last updated

In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes a group of objects that are treated the same way as a single instance of the same type of object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly. [1]

Contents

Overview

The Composite [2] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

Problems the Composite design pattern can solve

When defining (1) Part objects and (2) Whole objects that act as containers for Part objects, clients must treat them separately, which complicates client code. [3]

Solutions the Composite design pattern describes

This enables clients to work through the Component interface to treat Leaf and Composite objects uniformly: Leaf objects perform a request directly, and Composite objects forward the request to their child components recursively downwards the tree structure. This makes client classes easier to implement, change, test, and reuse.

See also the UML class and object diagram below.

Motivation

When dealing with Tree-structured data, programmers often have to discriminate between a leaf-node and a branch. This makes code more complex, and therefore, more error prone. The solution is an interface that allows treating complex and primitive objects uniformly. In object-oriented programming, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality. This is known as a "has-a" relationship between objects. [4] The key concept is that you can manipulate a single instance of the object just as you would manipulate a group of them. The operations you can perform on all the composite objects often have a least common denominator relationship. For example, if defining a system to portray grouped shapes on a screen, it would be useful to define resizing a group of shapes to have the same effect (in some sense) as resizing a single shape.

When to use

Composite should be used when clients ignore the difference between compositions of objects and individual objects. [1] If programmers find that they are using multiple objects in the same way, and often have nearly identical code to handle each of them, then composite is a good choice; it is less complex in this situation to treat primitives and composites as homogeneous.

Structure

UML class and object diagram

A sample UML class and object diagram for the Composite design pattern. W3sDesign Composite Design Pattern UML.jpg
A sample UML class and object diagram for the Composite design pattern.

In the above UML class diagram, the Client class doesn't refer to the Leaf and Composite classes directly (separately). Instead, the Client refers to the common Component interface and can treat Leaf and Composite uniformly.
The Leaf class has no children and implements the Component interface directly.
The Composite class maintains a container of child Component objects (children) and forwards requests to these children (for each child in children: child.operation()).

The object collaboration diagram shows the run-time interactions: In this example, the Client object sends a request to the top-level Composite object (of type Component) in the tree structure. The request is forwarded to (performed on) all child Component objects (Leaf and Composite objects) downwards the tree structure.

Defining Child-Related Operations
Defining child-related operations in the Composite design pattern. W3sDesign Composite Design Pattern Type Safety UML.jpg
Defining child-related operations in the Composite design pattern.

There are two design variants for defining and implementing child-related operations like adding/removing a child component to/from the container (add(child)/remove(child)) and accessing a child component (getChild()):

The GoF authors present a variant of the Composite design pattern that emphasizes transparency over type safety and discuss the tradeoffs of the two approaches. [1]

The type-safe approach is particularly palatable if the composite structure is fixed post construction: the construction code does not require transparency because it needs to know the types involved in order to construct the composite. If downstream, the code does not need to modify the structure, then the child manipulation operations do not need to be present on the Component interface.

UML class diagram

Composite pattern in UML. Composite UML class diagram (fixed).svg
Composite pattern in UML.
Component
Leaf
Composite
Composite pattern in LePUS3. Composite pattern in LePUS3.png
Composite pattern in LePUS3.

Variation

As it is described in Design Patterns, the pattern also involves including the child-manipulation methods in the main Component interface, not just the Composite subclass. More recent descriptions sometimes omit these methods. [7]

Example

This C++23 implementation is based on the pre C++98 implementation in the book.

importstd;usingCurrency=double;template<typenameT>usingLinkedList=std::list<T>;// declares the interface for objects in the composition.classEquipment{// Componentprivate:std::stringname;CurrencynetPrice;protected:Equipment()=default;Equipment(constString&name):name{name},netPrice{0}{}public:// implements default behavior for the interface common to all classes, as appropriate.[[nodiscard]]virtualconststd::string&getName()constnoexcept{returnname;}virtualvoidsetName(conststd::string&name_)noexcept{name=name_;}[[nodiscard]]virtualCurrencygetNetPrice()constnoexcept{returnnetPrice;}virtualvoidsetNetPrice(CurrencynetPrice_)noexcept{netPrice=netPrice_;}// declares an interface for accessing and managing its child components.virtualvoidadd(std::shared_ptr<Equipment>)=0;virtualvoidremove(std::shared_ptr<Equipment>)=0;virtual~Equipment()=default;};// defines behavior for components having children.classCompositeEquipment:publicEquipment{// Compositeprivate:// stores child components.LinkedList<std::shared_ptr<Equipment>>equipment;protected:CompositeEquipment()=default;CompositeEquipment(conststd::string&name):Equipment(name),equipment{}{}public:// implements child-related operations in the Component interface.[[nodiscard]]virtualCurrencygetNetPrice()constnoexceptoverride{Currencytotal=Equipment::getNetPrice();for(constEquipment&i:equipment){total+=i->getNetPrice();}returntotal;}virtualvoidadd(std::shared_ptr<Equipment>equipment_)override{equipment.push_front(equipment_.get());}virtualvoidremove(std::shared_ptr<Equipment>equipment_)override{equipment.remove(equipment_.get());}};// represents leaf objects in the composition.classFloppyDisk:publicEquipment{// Leafpublic:FloppyDisk(conststd::string&name_):Equipment(name){}// A leaf has no children.voidadd(std::shared_ptr<Equipment>)override{throwstd::runtime_error("FloppyDisk::add() cannot be called!");}voidremove(std::shared_ptr<Equipment>)override{throwstd::runtime_error("FloppyDisk::remove() cannot be called!");}};classChassis:publicCompositeEquipment{public:Chassis(conststd::string&name):CompositeEquipment(name){}};intmain(){// The smart pointers prevent memory leaks.std::shared_ptr<FloppyDisk>fd1=std::make_shared<FloppyDisk>("3.5in Floppy");fd1->setNetPrice(19.99);std::println("{}: netPrice = {}",fd1->getName(),fd1->getNetPrice);std::shared_ptr<FloppyDisk>fd2=std::make_shared<FloppyDisk>("5.25in Floppy");fd2->setNetPrice(29.99);std::println("{}: netPrice = {}",fd2->getName(),fd2->getNetPrice);std::unique_ptr<Chassis>ch=std::make_unique<Chassis>("PC Chassis");ch->setNetPrice(39.99);ch->add(fd1);ch->add(fd2);std::println("{}: netPrice = {}",ch->getName(),ch->getNetPrice);fd2->add(fd1);}

The program output is

3.5inFloppy:netPrice=19.995.25inFloppy:netPrice=29.99PCChassis:netPrice=89.97terminatecalledafterthrowinganinstanceof'std::runtime_error'what():FloppyDisk::add

See also

References

  1. 1 2 3 Gamma, Erich; Richard Helm; Ralph Johnson; John M. Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. pp.  395. ISBN   0-201-63361-2.
  2. Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp.  163ff. ISBN   0-201-63361-2.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. "The Composite design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12.
  4. Scott Walters (2004). Perl Design Patterns Book. Archived from the original on 2016-03-08. Retrieved 2010-01-18.
  5. "The Composite design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12.
  6. "The Composite design pattern - Implementation". w3sDesign.com. Retrieved 2017-08-12.
  7. Geary, David (13 September 2002). "A look at the Composite design pattern". Java Design Patterns. JavaWorld . Retrieved 2020-07-20.