Mechanism and overhead of RTTI

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Typically, RTTI is implemented by placing an additional pointer in a class s virtual function table. This pointer points to the type_info structure for that particular type. The effect of a typeid( ) expression is quite simple: the virtual function table pointer fetches the type_info pointer, and a reference to the resulting type_info structure is produced. Since this is just a two-pointer dereference operation, it is a constant time operation.

For a dynamic_cast<destination*>(source_pointer), most cases are quite straightforward: source_pointer s RTTI information is retrieved, and RTTI information for the type destination* is fetched. A library routine then determines whether source_pointer s type is of type destination* or a base class of destination*. The pointer it returns may be adjusted because of multiple inheritance if the base type isn t the first base of the derived class. The situation is more complicated with multiple inheritance because a base type may appear more than once in an inheritance hierarchy and virtual base classes are used.

Because the library routine used for dynamic_cast must check through a list of base classes, the overhead for dynamic_cast may be higher than typeid( ) (but you get different information, which may be essential to your solution), and it may take more time to discover a base class than a derived class. In addition, dynamic_cast compares any type to any other type; you aren t restricted to comparing types within the same hierarchy. This adds extra overhead to the library routine used by dynamic_cast.