Why size of classes is larger in case of virtual inheritance?

Question

Virtual base class is a way of preventing multiple instances of a given class appearing in an inheritance hierarchy when using multiple inheritance . Then for the following classes

class level0 {
    int a;
    public :
    level0();
};

class level10:virtual public level0 {
    int b;
    public :
    level10();
};

class level11 :virtual public level0 {
    int c;
    public :
    level11();
};

class level2 :public level10,public level11 { 
    int d;
    public:
    level2();
};

I got following sizes of the classes

size of level0 4

size of level10 12

size of level11 12

size of level2 24

but when I removed virtual from inheritance of level10 and level11 I got following output

sizeof level0 4

sizeof level10 8

sizeof level11 8

sizeof level2 20

If virtual inheritance prevents multiple instances of a base class, then why size of classes is greater in case of virtual inheritance?

You'll see the effect better if level0 is larger. As it is, its size gets lost in the noise. Give it 3 or 4 int data members and things will be clearer. — Pete Becker
@Marius nope, that question has one virtual, this one has two. The answer uses that single virtual as part of the size description: it wouldn't work as an answer to the above. — Yakk - Adam Nevraumont
Ok then duplicate of stackoverflow.com/questions/9737847/… — Marius
What architecture are you running this on? What is the size of a pointer vs. the size of an int? — David Rodríguez - dribeas

John Dibling John Dibling · Accepted Answer · 2013-06-24T19:14:06

Because when using virtual inheritence, the compiler will create* a vtable to point to the correct offsets for the various classes, and a pointer to that vtable is stored along with the class.

"Will create" -- vtables are not dictated by the Standard, but the behaviors implied by virtual inheritence is. Most compilers use vtables to implement the functionality dictated by the Standard.

Why size of classes is larger in case of virtual inheritance?

2 Answers