Question

Which virtual table will be pure virtual function located? In the base class or derived class?

For example, what does the virtual table look like in each class?

class Base {

  virtual void f() =0;
  virtual void g();
}


class Derived: public Base{

  virtual void f();
  virtual void g();

}

Answer 1

g++ -fdump-class-hierarchy layout.cpp produces a file layout.cpp.class. The content of layout.cpp.class will show the following:

Vtable for Base
Base::_ZTV4Base: 4u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI4Base)
16    __cxa_pure_virtual
24    Base::g

Class Base
   size=8 align=8
   base size=8 base align=8
Base (0x7ff893479af0) 0 nearly-empty
    vptr=((& Base::_ZTV4Base) + 16u)

Vtable for Derived
Derived::_ZTV7Derived: 4u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI7Derived)
16    Derived::f
24    Derived::g

Class Derived
   size=8 align=8
   base size=8 base align=8
Derived (0x7ff893479d90) 0 nearly-empty
    vptr=((& Derived::_ZTV7Derived) + 16u)
  Base (0x7ff893479e00) 0 nearly-empty
      primary-for Derived (0x7ff893479d90)

Removing the ‘pureness’ of f changes the fifth line to:

16    Base::f