Why doesn't storage for a duplicate empty base overlap with a vtable pointer?

Question

Consider this example:

#include <iostream>

int main()
{
    struct A {};
    struct B : A {};
    struct C : A, B {};

    std::cout << sizeof(A) << '\n'; // 1
    std::cout << sizeof(B) << '\n'; // 1
    std::cout << sizeof(C) << '\n'; // 2, because of a duplicate base

    struct E : A {virtual ~E() {}};
    struct F : A, B {virtual ~F() {}};

    std::cout << sizeof(E) << '\n'; // 8, the base overlaps the vtable pointer
    std::cout << sizeof(F) << '\n'; // 16, but why?
}

^{(run on godbolt)}

Here you can see that for struct E the empty base class (which is 1 byte large) uses the same storage as the vtable pointer, as expected.

But for struct F, which has a duplicate empty base, this doesn't happen. What causes this?

I get the same result on GCC, Clang, and MSVC. The results above are for x64, so sizeof(void *) == 8.

---

Interestingly, for struct G : A, B {void *ptr;}; GCC and Clang do perform EBO (the size is 8), but MSVC doesn't (the size is 16).

Answer 1

Because the compiler adds one byte padding after struct A

F {vptr(8) + 0 members from A + 1 padding (because A is empty)+0 from b} = 9 then the compiler add 7 bytes padding to align storage of the struct;

E {vptr(8) + 0 members for A} = 8 No padding required

from Microsoft

Every data object has an alignment-requirement. For structures, the requirement is the largest of its members. Every object is allocated an offset so that offset % alignment-requirement == 0

https://docs.microsoft.com/en-us/cpp/c-language/storage-and-alignment-of-structures?view=vs-2019

EDIT:

here is my demo:

int main()
{
    C c;
    A* a = &c;
    B* b = &c;

    std::cout << sizeof(A) << " " << a << '\n'; 
    std::cout << sizeof(B) << " " << b << '\n'; 
    std::cout << sizeof(C) << " " << &c << '\n'; 

    E e;
    a = &e;
    std::cout << sizeof(E) <<" " << &e << " " << a << '\n'; 

    F f;
    a = &f;
    b = &f;
    std::cout << sizeof(F) << " " << &f << " " << a << " " << b << '\n';

}

output:

1 0000007A45B7FBB4
1 0000007A45B7FBB5
1 0000007A45B7FBB4
8 0000007A45B7FC18 0000007A45B7FC20
16 0000007A45B7FC38 0000007A45B7FC40 0000007A45B7FC41

as you can see a & b never overlaps with each other and with vptr on multiple inheritance each has its own pointer value

note compiled by VC2019 x64 build