轉(zhuǎn)載自：
http://patmusing.blog.163.com/blog/static/135834960201002321018760/
在面向?qū)ο蟮南到y(tǒng)中，我們經(jīng)常會(huì)遇到一類具有“容器”特征的對(duì)象，即它們?cè)诔洚?dāng)對(duì)象的同時(shí)，又是其他對(duì)象的容器。
舉例：
在操作系統(tǒng)中，文件的概念很廣泛，其中文件可以是普通文件，也可以是目錄(在Unix中，設(shè)備也是文件)，目錄中可以存放文件。Composite設(shè)計(jì)模式就是將“客戶代碼與復(fù)雜的對(duì)象容器結(jié)構(gòu)”解耦，讓對(duì)象容器自己來實(shí)現(xiàn)自身的復(fù)雜結(jié)構(gòu)，從而使得客戶代碼就像處理簡單對(duì)象(文件)一樣來處理復(fù)雜的對(duì)象容器(目錄)。
“Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.” – GoF
調(diào)用Directory類對(duì)象的process函數(shù)，和調(diào)用PhysicalFile類對(duì)象的process一樣簡單。
從上面的UML類圖中，可以看出Directory和File這兩個(gè)類之間的關(guān)系：
1. Directory “is a”File
2. Directory “has a(more)” File
這是典型的遞歸結(jié)構(gòu)。因此在處理遞歸問題時(shí)，如果必要，可以考慮采用Composite模式。后面要講到的Decorator模式也是如此。
// Composite.h
#include <iostream>
#include <list>
using namespace std;
class File
{
public:
virtual void process() = 0;
// 虛函數(shù)：增加一個(gè)文件
virtual void add(File* file)
{
}
// 虛函數(shù)：刪除一個(gè)文件
virtual void remove(File* file)
{
}
public:
virtual ~File()
{
cout << "in the destructor of File..." << endl;
}
};
// 葉子節(jié)點(diǎn)
class PhysicalFile : public File
{
public:
void process()
{
cout << "process() in PhysicalFile..." << endl;
}
public:
~PhysicalFile()
{
cout << "in the destructor of PhysicalFile..." << endl;
}
};
// 容器節(jié)點(diǎn)：Composite節(jié)點(diǎn)
class Directory : public File
{
private:
list<File*> file_list;
public:
Directory()
{
}
void process()
{
cout << "process() in Directory..." << endl;
if(!file_list.empty())
{
for(list<File*>::iterator it = file_list.begin(); it != file_list.end(); it++)
{
File* f = *it;
f->process();
}
}
}
void add(File* file)
{
file_list.push_back(file);
}
void remove(File* file)
{
file_list.remove(file);
}
public:
~Directory()
{
cout << "in the destructor of Directory..." << endl;
}
};
// Composite.cpp
#include "Composite.h"
int main(int argc, char **argv)
{
File *f1 = new Directory;
File *f2 = new Directory;
File *f3 = new PhysicalFile;
f2->add(f3);
f1->add(f2);
File *f4 = new Directory;
File *f5 = new Directory;
File *f6 = new Directory;
File *f7 = new PhysicalFile;
f6->add(f7);
f5->add(f6);
f4->add(f5);
f1->add(f4);
f1->process();
f1->remove(f4);
cout << "+++++++++++++++++++++++" << endl;
f1->process();
// STL container中的元素是指針對(duì)象，那么必須手動(dòng)刪除。
delete f1;
delete f2;
delete f3;
delete f4;
delete f5;
delete f6;
delete f7;
return 0;
}
上述程序中，各對(duì)象之間的關(guān)系如下圖：
其中f3和f7為PhysicalFile對(duì)象。f1包含了f2和f4，f2包含了f3，f4包含了f5，f5包含了f6，f6包含了f7。
運(yùn)行結(jié)果如下：
process() in Directory... // f1
process() in Directory... // f2
process() in PhysicalFile... // f3
process() in Directory... // f4
process() in Directory... // f5
process() in Directory... // f6
process() in PhysicalFile... // f7
+++++++++++++++++++++++ // 刪除f4后的輸出(可以看到f4及其包含的對(duì)象全部被刪除了)
process() in Directory... // f1
process() in Directory... // f2
process() in PhysicalFile... // f3
in the destructor of Directory...
in the destructor of File...
in the destructor of Directory...
in the destructor of File...
in the destructor of PhysicalFile...
in the destructor of File...
in the destructor of Directory...
in the destructor of File...
in the destructor of Directory...
in the destructor of File...
in the destructor of Directory...
in the destructor of File...
in the destructor of PhysicalFile...
in the destructor of File...