用C++模拟C#的event机制
2005-11-29 20:51 Orin 阅读(1009) 评论(5) 编辑 收藏 举报本文章为farproc.com站长袁晓辉(海洋)的原创文章,作者保留所有权利。如果转载,请务必注明来自www.farproc.com!
由于C++不支持event关键字,我们就必须自己写代码。在这里我通过模板类的手段来实现,因为该手段实现的效果和C#比较类似。
在VC6中新建一个win32 console app,命名为“cppevent“。新建一个.h头文件,命名为“event.h”,代码如下:
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为了能够在定义是指定事件处理函数的原型,我使用了template,为了能和C#一样用+=和-=来定制和撤消事件,我重载了这两个操作符(C#不支持操作符重载),为了能像C#一样直接把event当做函数调用,我有重载了Handler自定义转换操作符,可惜的是,这一点模拟得不是很像,在调用时还必须来一次强制转换才可以:(,具体参看后面的代码:
C++版的MyClass如下:
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和C#版的MyClass比较一下你就会发现代码非常接近,当然,C#是在语言级直接支持event关键字的,而C++不支持,用模板类代替,所以声明事件的代码有些不一样。还有就是FireEvent()中C++不能把event对象直接当做函数来调用,多了强制类型转换。
C++版的客户代码如下:
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我们可以看到,可C#版的客户代码相比,核心部分是非常接近的,我们已经可以和C#一样用“+=”和“-=”来定制事件和撤消事件定制,并在Obj的FireEvent()被调用时收到事件通知,输出文本。
鉴于篇幅的原因,我们没有仔细比较两个版本的event的add和remove访问器/成员函数,其实二者也是非常类似的,你可以自己试试。C++版的event的add和remove均为virtual的,你可以从event类继承出来一个MyEvent类,然后重新实现这两个函数,就可以定制自己的add和remove了。这和C#的add/remove访问器的也是非常相像的。
三、总结
通过这场“模仿show”我们可以从更深的层次理解C#的event机制,更重要的是我们用自己所熟悉的东西(C++,模板类)来模仿并解释了我们目前还不太熟悉的东西(C#,event)。
其实,C#的delegate就是C++的函数指针,C# event的核心机制就是C++中的模板(定义event时表现出来)和运算符重载(+=、-=和直接把event当做函数调用)的结合体。C#把C++中容易出错的部分用“新特性”封装了起来,把这部分工作从programmer身上转移到了compiler身上,让我们把更多的精力集中到业务逻辑的处理上。