Wednesday, June 25, 2008

follow the vptr - RTTI (dynamic_cast, type_info...

This is yet another blog post on vtbl, RTTI, dynamic_cast, type_info...

We won't be too far off if we imagine that all RTTI operations work their magic by following the vptr.

Remember each polymorphic class has a distinct vtbl. If there are 521 polymorphic classes in memory, then there are exactly that many v-tables in memory. Each polymorphic object holds a 32-bit vptr seated at the vtbl of the "real" class. Recall that during subclass instantiation, the vptr first gets seated at the parent vtbl, then reseated at the subclass vtbl.

Now you see that if there's no virtual function among base and subclasses, then there's no vtbl/vptr, and RTTI can't "follow the vptr".

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