C++ vtable query

Question

I have a query in regard to the explaination provided here http://www.parashift.com/c++-faq/virtual-functions.html#faq-20.4

In the sample code the function mycode(Base *p), calls virt3 method as p->virt3(). Here how exactly the compiler know that virt3 is found in third slot of vtable? How do it compare and with what ?

Answer 1

When the compiler sees the definition of Base it decides the layout of its vtable according to some algorithm¹, which is common to all its derived classes as far as methods inherited from Base are concerned (derived classes may add other virtual methods, but they are put into the vtable after the stuff inherited from Base).

Thus, when the compiler sees p->virt3(), it already knows that for any object that inherits from Base the pointer to the correct virt3 is e.g. in the third slot of the vtable (because that's how it laid out the vtable of Base at the moment of its definition), so it can correctly generate the code for the virtual call.

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Long story short (driving inspiration from @David Rodríguez's comment): it knows where it stays because he decided it before.

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1. The standard do not mandate any particular algorithm (actually, it doesn't say anything about how the C++ ABI should be implenented), but there are several widespread C++ ABI specifications, notably the COM ABI on Windows and the Itanium ABI on Linux (and in general for gcc). Obviously, given the same class definition, the algorithm must give the same vtable layout every time, otherwise it would be impossible to link together different object modules.