Tham khảo tài liệu 'thinking in cplus plus (p27)', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | want to know the exact type it is pointing to you must dereference the pointer. For example if s is a Shape cout typeid s .name endl will print out the type of the object s points to. You can also ask a typeinfo object if it precedes another typeinfo object in the implementation-defined collation sequence using before typeinfo which returns true or false. When you say if typeid me .before typeid you . you re asking if me occurs before you in the collation sequence. The second syntax for RTTI is called a type-safe downcast. The reason for the term downcast is again the historical arrangement of the class hierarchy diagram. If casting a Circle to a Shape is an upcast then casting a Shape to a Circle is a downcast. However you know a Circle is also a Shape and the compiler freely allows an upcast assignment but you don t know that a Shape is necessarily a Circle so the compiler doesn t allow you to perform a downcast assignment without using an explicit cast. You can of course force your way through using ordinary C-style casts or a C static_cast described at the end of this chapter which says I hope this is actually a Circle and I m going to pretend it is. Without some explicit knowledge that it is in fact a Circle this is a totally dangerous thing to do. A common approach in vendor-defined RTTI is to create some function that attempts to assign for this example a Shape to a Circle checking the type in the process. If this function returns the address it was successful if it returns null you didn t have a Circle . The C RTTI typesafe-downcast follows this attempt-to-cast function form but it uses very logically the template syntax to produce the special function dynamic_cast. So the example becomes Shape sp new Circle Circle cp dynamic_cast Circle sp if cp cout cast successful The template argument for dynamic_cast is the type you want the function to produce and this is the return value for the function. The function argument is what you are trying to cast from. .
