深入剖析设计模式中的组合模式应用及在C++中的实现

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：