c++回调函数 callback

浏览数：49 / 时间：2015年06月08日

（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：

typedef?void?(__stdcall?*DownloadCallback)(const?char*?pURL,?bool?bOK);??
void?DownloadFile(const?char*?pURL,?DownloadCallback?callback)??
{??
????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
????callback(pURL,?true);??
}??
void?__stdcall?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
{??
????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
}??

(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:

class?IDownloadSink??
{??
public:??
????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)?=?0;??
};??
?? ?
?? ?
class?CMyDownloader??
{??
public:??
????CMyDownloader(IDownloadSink*?pSink)??
????????:m_pSink(pSink)??
????{??
????}??
?? ?
????void?DownloadFile(const?char*?pURL)??
????{??
????????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
????????if(m_pSink?!=?NULL)??
????????{??
????????????m_pSink->OnDownloadFinished(pURL,?true);??
????????}??
????}??
?? ?
private:??
????IDownloadSink*?m_pSink;??
};??
?? ?
class?CMyFile:?public?IDownloadSink??
{??
public:??
????void?download()??
????{??
????????CMyDownloader?downloader(this);??
????????downloader.DownloadFile("www.baidu.com");??
????}??
?? ?
????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
????{??
????????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
????}??
};??

? ?

(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:

class?CDownloadDelegateBase??
{??
public:??
????virtual?void?Fire(const?char*?pURL,?bool?bOK)?=?0;??
};??
?? ?
template<typename?O,?typename?T>??
class?CDownloadDelegate:?public?CDownloadDelegateBase??
{??
????typedef?void?(T::*Fun)(const?char*,?bool);??
public:??
????CDownloadDelegate(O*?pObj?=?NULL,?Fun?pFun?=?NULL)??
????????:m_pFun(pFun),?m_pObj(pObj)??
????{??
????}??
????? ?
????virtual?void?Fire(const?char*?pURL,?bool?bOK)??
????{??
????????if(m_pFun?!=?NULL??
????????????&&?m_pObj?!=?NULL)??
????????{??
????????????(m_pObj->*m_pFun)(pURL,?bOK);??
????????}??
????}??
?? ?
private:??
????Fun?m_pFun;??
????O*?m_pObj;??
};??
?? ?
template<typename?O,?typename?T>??
CDownloadDelegate<O,T>*?MakeDelegate(O*?pObject,?void?(T::*pFun)(const?char*?pURL,?bool))??
{??
????return?new?CDownloadDelegate<O,?T>(pObject,?pFun);??
}??
?? ?
class?CDownloadEvent??
{??
public:??
????~CDownloadEvent()??
????{??
????????vector<CDownloadDelegateBase*>::iterator?itr?=?m_arDelegates.begin();??
????????while?(itr?!=?m_arDelegates.end())??
????????{??
????????????delete?*itr;??
????????????++itr;??
????????}??
????????m_arDelegates.clear();??
????}??
?? ?
????void?operator?+=?(CDownloadDelegateBase*?p)??
????{??
????????m_arDelegates.push_back(p);??
????}??
?? ?
????void?operator?-=?(CDownloadDelegateBase*?p)??
????{??
????????ITR?itr?=?remove(m_arDelegates.begin(),?m_arDelegates.end(),?p);??
?? ?
????????ITR?itrTemp?=?itr;??
????????while?(itrTemp?!=?m_arDelegates.end())??
????????{??
????????????delete?*itr;??
????????????++itr;??
????????}??
????????m_arDelegates.erase(itr,?m_arDelegates.end());??
????}??
?? ?
????void?operator()(const?char*?pURL,?bool?bOK)??
????{??
????????ITR?itrTemp?=?m_arDelegates.begin();??
????????while?(itrTemp?!=?m_arDelegates.end())??
????????{??
????????????(*itrTemp)->Fire(pURL,?bOK);??
????????????++itrTemp;??
????????}??
????}??
?? ?
private:??
????vector<CDownloadDelegateBase*>?m_arDelegates;??
????typedef?vector<CDownloadDelegateBase*>::iterator?ITR;??
};??
?? ?
?? ?
class?CMyDownloaderEx??
{??
public:??
????void?DownloadFile(const?char*?pURL)??
????{??
????????cout?<<?"downloading:?"?<<?pURL?<<?""?<<?endl;??
????????downloadEvent(pURL,?true);??
????}??
?? ?
????CDownloadEvent?downloadEvent;??
};??
?? ?
class?CMyFileEx??
{??
public:??
????void?download()??
????{??
????????CMyDownloaderEx?downloader;??
????????downloader.downloadEvent?+=?MakeDelegate(this,?&CMyFileEx::OnDownloadFinished);??
????????downloader.DownloadFile("www.baidu.com");??
????}??
?? ?
????virtual?void?OnDownloadFinished(const?char*?pURL,?bool?bOK)??
????{??
????????cout?<<?"OnDownloadFinished,?URL:"?<<?pURL?<<?"????status:"?<<?bOK?<<?endl;??
????}??
};??

? ?

??? 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

? ?

我们可以用下面的代码测试我们上面的实现：

[cpp]?view plain copy

int?_tmain(int?argc,?_TCHAR*?argv[])??
{??
?? ?
????DownloadFile("www.baidu.com",?OnDownloadFinished);??
?? ?
????CMyFile?f1;??
????f1.download();??
?? ?
????CMyFileEx?ff;??
????ff.download();??
?? ?
????system("pause");??
?? ?
????return?0;??
}??

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。