C++ (Virtual) Function Table

Question

Is the C++ vtable only used for determining which section of code should be executed when a call is made to a virtual function, or does it have other uses at run-time?

On wikipedia it lists "dynamic dispatch" as a reason, but doesn't go into further details for C++.....

Answer 1

Some implementations place a pointer to information used by RTTI code ( typeid , dynamic_cast , ...) in a slot of the vtable (see here about the Itanium ABI , on which the g++ ABI is based).

_{(actually, if you read the Itanium ABI specification it's slightly more convoluted, but this is the general idea)}

Also, as you can see from the linked document, there's some more stuff to access the data of the base class, perform pointer adjustment when calling base-class methods, ..., mostly stuff needed in "complicated" object hierarchies (multiple inheritance, virtual base classes, ...), as in the classic single inheritance scenario the layout of a derived class is straightforward and doesn't require all this fuss.

Answer 2

The C++ vtable is simply a lookup table for function pointers. Any specific C++ implementation may use the vtable for other things; but that is implementation (and version) specific. Relying on the vtable for anything other than C++ to invoke your virtual methods is a really bad idea.