If at any time a resource acquires the right property with a value of the right class, it automatically becomes a member of this virtual class, without needing to be explicitly stated as such.
如果在任何时候资源获得具有适当类值的适当特性，那么无需显式声明，它会自动成为这一虚拟类的成员。
While some schools let students complete tasks at their own pace, others require that students participate in virtual class sessions and have specific deadlines for assignments.
有些学校会让学生按照自己的进度完成学业，而有些学校要求学生参加虚拟课堂，并且对完成学习任务有明确的期限。
A virtual class or function is code that the application will eventually need, but it isn't written yet.
所谓虚类或虚函数是指应用程序以后需要，但是目前还没写的代码。
A virtual class or function is code that the application will eventually need, but it isn't written yet.
所谓虚类或虚函数是指应用程序以后需要，但是目前还没写的代码。
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