Question

Suppose I have:

struct Foo: public Bar {
 ....
}

Foo introduces no new member varaibles. Foo only introduces a bunch of member functions & static functions. Does any part of the C++ standard now guarantee me that:

sizeof(Foo) == sizeof(Bar)

?

Thanks!

Answer 1

I could think of at least one scenario where sizeof(Foo) != sizeof(Bar):

class Bar {
public:
 int m_member;
};

class Foo : Bar {
    virtual ~Foo();
};

Foo will have a vtable pointer whilst Bar wouldn’t and the former’s size will be one word bigger that Bar. On 32bit DEBUG MSVC2010:

sizeof(Foo) - 8
sizeof(Bar) - 4

EDIT This holds true for structs as well as classes, I reran the test to confirm that.