Virtual Classes having more size

Question

class abstract 
{double abs; };

class A : virtual public abstract
{ double var; };

class B : virtual public abstract
{ double var; };

class derived : public A, public B
{ double der; };

sizeof(abstract) = 8

sizeof(A) = 24

sizeof(B) = 24

sizeof(derived) = 48

Why does this happen?

(Note: On the contrary without the virtual classes the sizes returned are 8, 16, 16, 40 respectively. )

**Probably** (but it's implementation defined) they're to keep track (point to) virtual base class. See also [this post](http://stackoverflow.com/questions/9737847/multiple-inheritance-size-of-class-for-virtual-pointers/9737936#9737936). — Adriano Repetti, Jul 10 '14 at 15:18

score 0 · Answer 1 · answered Jul 10 '14 at 15:16

0

It requires the type to store more information to facilitate virtual behavior.

answered Jul 10 '14 at 15:16

wbennett

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12

Sorry, I don't follow. – Level 31 Jul 10 '14 at 15:17
Sorry, there is no type information in any instance! And virtual functions have nothing to do with rtti! – Klaus Jul 10 '14 at 15:27
It depends on the compiler implementation, but it will store a pointer back to the vtable, and that is "more information". – wbennett Jul 10 '14 at 15:42

Virtual Classes having more size

1 Answers1