复杂的dynamic_cast在c

If C is the class type to which T points or refers,the run-time
check logically executes as follows:

• If,in the most derived object pointed (referred) to by v,v points (refers) to a public base class subobject of a C object [..]

• Otherwise,if v points (refers) to a public base class subobject of the most derived object,and the type of the most derived object
  has a base class,of type C,that is unambiguous and public,the
  result points (refers) to the C subobject of the most derived
  object.

即使没有关于Foo在包含演员的翻译单元中的存在的信息,这将会工作.

你也应该看看this question.