c++回调函数 callback

（1）Callback方式
Callback的本质是设置一个函数指针进去，然后在需要需要触发某个事件时调用该方法, 比如Windows的窗口消息处理函数就是这种类型。比如下面的示例代码，我们在Download完成时需要触发一个通知外面的事件：

?

1. typedef?void?(__stdcall?*DownloadCallback)(const?char*?pURL,?bool?bOK);??
2. void?DownloadFile(const?char*?pURL,?DownloadCallback?callback)??
3. {??
4. ????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
5. ????callback(pURL,?true);??
6. }??
7. void?__stdcall?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
8. {??
9. ????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
10. }??

(2)Sink方式
Sink的本质是你按照对方要求实现一个C++接口，然后把你实现的接口设置给对方，对方需要触发事件时调用该接口， COM中连接点就是居于这种方式。上面下载文件的需求，如果用Sink实现，代码如下:

1. class?IDownloadSink??
2. {??
3. public:??
4. ????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)?=?0;??
5. };??
6. ?? ?
7. ?? ?
8. class?CMyDownloader??
9. {??
10. public:??
11. ????CMyDownloader(IDownloadSink*?pSink)??
12. ????????:m_pSink(pSink)??
13. ????{??
14. ????}??
15. ?? ?
16. ????void?DownloadFile(const?char*?pURL)??
17. ????{??
18. ????????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
19. ????????if(m_pSink?!=?NULL)??
20. ????????{??
21. ????????????m_pSink->OnDownloadFinished(pURL,?true);??
22. ????????}??
23. ????}??
24. ?? ?
25. private:??
26. ????IDownloadSink*?m_pSink;??
27. };??
28. ?? ?
29. class?CMyFile:?public?IDownloadSink??
30. {??
31. public:??
32. ????void?download()??
33. ????{??
34. ????????CMyDownloader?downloader(this);??
35. ????????downloader.DownloadFile("www.baidu.com");??
36. ????}??
37. ?? ?
38. ????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
39. ????{??
40. ????????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
41. ????}??
42. };??

? ?

(3)Delegate方式
??? Delegate的本质是设置成员函数指针给对方，然后让对方在需要触发事件时调用。C#中用Delegate的方式实现Event，让C++程序员很是羡慕，C++中因为语言本身的关系，要实现Delegate还是很麻烦的。上面的例子我们用Delegate的方式实现如下:

1. class?CDownloadDelegateBase??
2. {??
3. public:??
4. ????virtual?void?Fire(const?char*?pURL,?bool?bOK)?=?0;??
5. };??
6. ?? ?
7. template<typename?O,?typename?T>??
8. class?CDownloadDelegate:?public?CDownloadDelegateBase??
9. {??
10. ????typedef?void?(T::*Fun)(const?char*,?bool);??
11. public:??
12. ????CDownloadDelegate(O*?pObj?=?NULL,?Fun?pFun?=?NULL)??
13. ????????:m_pFun(pFun),?m_pObj(pObj)??
14. ????{??
15. ????}??
16. ????? ?
17. ????virtual?void?Fire(const?char*?pURL,?bool?bOK)??
18. ????{??
19. ????????if(m_pFun?!=?NULL??
20. ????????????&&?m_pObj?!=?NULL)??
21. ????????{??
22. ????????????(m_pObj->*m_pFun)(pURL,?bOK);??
23. ????????}??
24. ????}??
25. ?? ?
26. private:??
27. ????Fun?m_pFun;??
28. ????O*?m_pObj;??
29. };??
30. ?? ?
31. template<typename?O,?typename?T>??
32. CDownloadDelegate<O,T>*?MakeDelegate(O*?pObject,?void?(T::*pFun)(const?char*?pURL,?bool))??
33. {??
34. ????return?new?CDownloadDelegate<O,?T>(pObject,?pFun);??
35. }??
36. ?? ?
37. class?CDownloadEvent??
38. {??
39. public:??
40. ????~CDownloadEvent()??
41. ????{??
42. ????????vector<CDownloadDelegateBase*>::iterator?itr?=?m_arDelegates.begin();??
43. ????????while?(itr?!=?m_arDelegates.end())??
44. ????????{??
45. ????????????delete?*itr;??
46. ????????????++itr;??
47. ????????}??
48. ????????m_arDelegates.clear();??
49. ????}??
50. ?? ?
51. ????void?operator?+=?(CDownloadDelegateBase*?p)??
52. ????{??
53. ????????m_arDelegates.push_back(p);??
54. ????}??
55. ?? ?
56. ????void?operator?-=?(CDownloadDelegateBase*?p)??
57. ????{??
58. ????????ITR?itr?=?remove(m_arDelegates.begin(),?m_arDelegates.end(),?p);??
59. ?? ?
60. ????????ITR?itrTemp?=?itr;??
61. ????????while?(itrTemp?!=?m_arDelegates.end())??
62. ????????{??
63. ????????????delete?*itr;??
64. ????????????++itr;??
65. ????????}??
66. ????????m_arDelegates.erase(itr,?m_arDelegates.end());??
67. ????}??
68. ?? ?
69. ????void?operator()(const?char*?pURL,?bool?bOK)??
70. ????{??
71. ????????ITR?itrTemp?=?m_arDelegates.begin();??
72. ????????while?(itrTemp?!=?m_arDelegates.end())??
73. ????????{??
74. ????????????(*itrTemp)->Fire(pURL,?bOK);??
75. ????????????++itrTemp;??
76. ????????}??
77. ????}??
78. ?? ?
79. private:??
80. ????vector<CDownloadDelegateBase*>?m_arDelegates;??
81. ????typedef?vector<CDownloadDelegateBase*>::iterator?ITR;??
82. };??
83. ?? ?
84. ?? ?
85. class?CMyDownloaderEx??
86. {??
87. public:??
88. ????void?DownloadFile(const?char*?pURL)??
89. ????{??
90. ????????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
91. ????????downloadEvent(pURL,?true);??
92. ????}??
93. ?? ?
94. ????CDownloadEvent?downloadEvent;??
95. };??
96. ?? ?
97. class?CMyFileEx??
98. {??
99. public:??
100. ????void?download()??
101. ????{??
102. ????????CMyDownloaderEx?downloader;??
103. ????????downloader.downloadEvent?+=?MakeDelegate(this,?&CMyFileEx::OnDownloadFinished);??
104. ????????downloader.DownloadFile("www.baidu.com");??
105. ????}??
106. ?? ?
107. ????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
108. ????{??
109. ????????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
110. ????}??
111. };??

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??? 可以看到Delegate的方式代码量比上面其他2种方式大多了，并且我们上面是固定参数数量和类型的实现方式，如果要实现可变参数，要更加麻烦的多。可变参数的方式可以参考这2种实现:

Yet Another C#-style Delegate Class in Standard C++
Member Function Pointers and the Fastest Possible C++ Delegates

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我们可以用下面的代码测试我们上面的实现：

[cpp]?view plaincopy

1. int?_tmain(int?argc,?_TCHAR*?argv[])??
2. {??
3. ?? ?
4. ????DownloadFile("www.baidu.com",?OnDownloadFinished);??
5. ?? ?
6. ????CMyFile?f1;??
7. ????f1.download();??
8. ?? ?
9. ????CMyFileEx?ff;??
10. ????ff.download();??
11. ?? ?
12. ????system("pause");??
13. ?? ?
14. ????return?0;??
15. }??

最后简单比较下上面3种实现回调的方法:

第一种Callback的方法是面向过程的，使用简单而且灵活，正如C语言本身。
第二种Sink的方法是面向对象的，在C++里使用较多， 可以在一个Sink里封装一组回调接口，适用于一系列比较固定的回调事件。
第三种Delegate的方法也是面向对象的，和Sink封装一组接口不同，Delegate的封装是以函数为单位，粒度比Sink更小更灵活。

你更倾向于用哪种方式来实现回调?

c++回调函数 callback

原文：http://www.cnblogs.com/poissonnotes/p/4396082.html