组合模式将对象组合成树形结构以表示“部分-整体”的层次结构。C o m p o s i t e 使得用户对单个对象和组合对象的使用具有一致性。

模式图：

适用场景：

你想表示对象的部分-整体层次结构。 你希望用户忽略组合对象与单个对象的不同，用户将统一地使用组合结构中的所有对象。

举例：

namespace FactoryMethod_DesignPattern { using System; using System.Collections; abstract class Component { protected string strName; public Component(string name) { strName = name; } abstract public void Add(Component c); public abstract void DumpContents(); // other operations for delete, get, etc. } class Composite : Component { private ArrayList ComponentList = new ArrayList(); public Composite(string s) : base(s) {} override public void Add(Component c) { ComponentList.Add(c); } public override void DumpContents() { // First dump the name of this composite node Console.WriteLine("Node: {0}", strName); // Then loop through children, and get then to dump their contents foreach (Component c in ComponentList) { c.DumpContents(); } } } class Leaf : Component { public Leaf(string s) : base(s) {} override public void Add(Component c) { Console.WriteLine("Cannot add to a leaf"); } public override void DumpContents() { Console.WriteLine("Node: {0}", strName); } } /// <summary> /// Summary description for Client. /// </summary> public class Client { Component SetupTree() { // here we have to create a tree structure, // consisting of composites and leafs. Composite root = new Composite("root-composite"); Composite parentcomposite; Composite composite; Leaf leaf; parentcomposite = root; composite = new Composite("first level - first sibling - composite"); parentcomposite.Add(composite); leaf = new Leaf("first level - second sibling - leaf"); parentcomposite.Add(leaf); parentcomposite = composite; composite = new Composite("second level - first sibling - composite"); parentcomposite.Add(composite); composite = new Composite("second level - second sibling - composite"); parentcomposite.Add(composite); // we will leaf the second level - first sibling empty, and start // populating the second level - second sibling parentcomposite = composite; leaf = new Leaf("third level - first sibling - leaf"); parentcomposite.Add(leaf); leaf = new Leaf("third level - second sibling - leaf"); parentcomposite.Add(leaf); composite = new Composite("third level - third sibling - composite"); parentcomposite.Add(composite); return root; } public static int Main(string[] args) { Component component; Client c = new Client(); component = c.SetupTree(); component.DumpContents(); return 0; } } }

可以看出，Composite类型的对象可以包含其它Component类型的对象。换而言之，Composite类型对象可以含有其它的树枝（Composite）类型或树叶（Leaf）类型的对象。

合成模式的实现根据所实现接口的区别分为两种形式，分别称为安全模式和透明模式。合成模式可以不提供父对象的管理方法，但合成模式必须在合适的地方提供子对象的管理方法（诸如：add、remove、getChild等）。

透明方式

作为第一种选择，在Component里面声明所有的用来管理子类对象的方法，包括add（）、remove（），以及getChild（）方法。这样做的好处是所有的构件类都有相同的接口。在客户端看来，树叶类对象与合成类对象的区别起码在接口层次上消失了，客户端可以同等同的对待所有的对象。这就是透明形式的合成模式。

这个选择的缺点是不够安全，因为树叶类对象和合成类对象在本质上是有区别的。树叶类对象不可能有下一个层次的对象，因此add（）、remove（）以及getChild（）方法没有意义，是在编译时期不会出错，而只会在运行时期才会出错。

安全方式

第二种选择是在Composite类里面声明所有的用来管理子类对象的方法。这样的做法是安全的做法，因为树叶类型的对象根本就没有管理子类对象的方法，因此，如果客户端对树叶类对象使用这些方法时，程序会在编译时期出错。

这个选择的缺点是不够透明，因为树叶类和合成类将具有不同的接口。

这两个形式各有优缺点，需要根据软件的具体情况做出取舍决定。

安全式的合成模式实现: 只有composite有Add ,remove,delete等方法.

以下示例性代码演示了安全式的合成模式代码：

// Composite pattern -- Structural example using System; using System.Text; using System.Collections; // "Component" abstract class Component { // Fields protected string name; // Constructors public Component( string name ) { this.name = name; } // Operation public abstract void Display( int depth ); } // "Composite" class Composite : Component { // Fields private ArrayList children = new ArrayList(); // Constructors public Composite( string name ) : base( name ) {} // Methods public void Add( Component component ) { children.Add( component ); } public void Remove( Component component ) { children.Remove( component ); } public override void Display( int depth ) { Console.WriteLine( new String( '-', depth ) + name ); // Display each of the node's children foreach( Component component in children ) component.Display( depth + 2 ); } } // "Leaf" class Leaf : Component { // Constructors public Leaf( string name ) : base( name ) {} // Methods public override void Display( int depth ) { Console.WriteLine( new String( '-', depth ) + name ); } } /// <summary> /// Client test /// </summary> public class Client { public static void Main( string[] args ) { // Create a tree structure Composite root = new Composite( "root" ); root.Add( new Leaf( "Leaf A" )); root.Add( new Leaf( "Leaf B" )); Composite comp = new Composite( "Composite X" ); comp.Add( new Leaf( "Leaf XA" ) ); comp.Add( new Leaf( "Leaf XB" ) ); root.Add( comp ); root.Add( new Leaf( "Leaf C" )); // Add and remove a leaf Leaf l = new Leaf( "Leaf D" ); root.Add( l ); root.Remove( l ); // Recursively display nodes root.Display( 1 ); } }

透明式的合成模式实现: 每个里都有add,remove等修改方法.

以下示例性代码演示了安全式的合成模式代码：

// Composite pattern -- Structural example using System; using System.Text; using System.Collections; // "Component" abstract class Component { // Fields protected string name; // Constructors public Component( string name ) { this.name = name; } // Methods abstract public void Add(Component c); abstract public void Remove( Component c ); abstract public void Display( int depth ); } // "Composite" class Composite : Component { // Fields private ArrayList children = new ArrayList(); // Constructors public Composite( string name ) : base( name ) {} // Methods public override void Add( Component component ) { children.Add( component ); } public override void Remove( Component component ) { children.Remove( component ); } public override void Display( int depth ) { Console.WriteLine( new String( '-', depth ) + name ); // Display each of the node's children foreach( Component component in children ) component.Display( depth + 2 ); } } // "Leaf" class Leaf : Component { // Constructors public Leaf( string name ) : base( name ) {} // Methods public override void Add( Component c ) { Console.WriteLine("Cannot add to a leaf"); } public override void Remove( Component c ) { Console.WriteLine("Cannot remove from a leaf"); } public override void Display( int depth ) { Console.WriteLine( new String( '-', depth ) + name ); } } /// <summary> /// Client test /// </summary> public class Client { public static void Main( string[] args ) { // Create a tree structure Composite root = new Composite( "root" ); root.Add( new Leaf( "Leaf A" )); root.Add( new Leaf( "Leaf B" )); Composite comp = new Composite( "Composite X" ); comp.Add( new Leaf( "Leaf XA" ) ); comp.Add( new Leaf( "Leaf XB" ) ); root.Add( comp ); root.Add( new Leaf( "Leaf C" )); // Add and remove a leaf Leaf l = new Leaf( "Leaf D" ); root.Add( l ); root.Remove( l ); // Recursively display nodes root.Display( 1 ); } }

实例

再看看一个完整些的例子：

#include <iostream> #include <string> #include <list> using namespace std; class Component { protected: string name; public: Component(string name) :name(name) { } virtual void AddComponent(Component *component) { } virtual void RemoveComponent(Component *component) { } virtual void GetChild(int depth) { } }; class Leaf: public Component { public: Leaf(string name) :Component(name) { } void AddComponent(Component *component) { cout<<"Leaf can't add component"<<endl; } void RemoveComponent(Component *component) { cout<<"Leaf can't remove component"<<endl; } void GetChild(int depth) { string _tmpstring(depth, '-'); cout<<_tmpstring<<name<<endl; } }; class Composite:public Component { private: list<Component*> _componets; public: Composite(string name) :Component(name) { } void AddComponent(Component *component) { _componets.push_back(component); } void RemoveComponent(Component *component) { _componets.remove(component); } void GetChild(int depth) { string tmpstring (depth, '-'); cout<<tmpstring<<name<<endl; list<Component*>::iterator iter = _componets.begin(); for(; iter != _componets.end(); iter++) { (*iter)->GetChild(depth + 2); } } }; int main() { Composite *root = new Composite("root"); Leaf *leaf1 = new Leaf("leaf1"); Leaf *leaf2 = new Leaf("leaf2"); root->AddComponent(leaf1); root->AddComponent(leaf2); Composite *lay2 = new Composite("layer2"); Leaf *leaf4 = new Leaf("leaf4"); lay2->AddComponent(leaf4); Composite *lay1 = new Composite("layer1"); Leaf *leaf3 = new Leaf("leaf3"); lay1->AddComponent(leaf3); lay1->AddComponent(lay2); root->AddComponent(lay1); root->GetChild(1); cout<<endl; lay1->GetChild(1); cout<<endl; lay2->GetChild(1); delete root; delete lay1; delete lay2; delete leaf1; delete leaf2; delete leaf3; delete leaf4; system("pause"); return 0; }

输出：